Draft ECMA-262 / July 28, 2021

ECMAScript® 2022 Language Specification

About this Specification

The document at https://tc39.es/ecma262/ is the most accurate and up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any finished proposals (those that have reached Stage 4 in the proposal process and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.

This document is available as a single page and as multiple pages.

Contributing to this Specification

This specification is developed on GitHub with the help of the ECMAScript community. There are a number of ways to contribute to the development of this specification:

Refer to thecolophonfor more information on how this document is created.

Introduction

This Ecma Standard defines the ECMAScript 2022 Language. It is the twelfth edition of the ECMAScript Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one of the world's most widely used general-purpose programming languages. It is best known as the language embedded in web browsers but has also been widely adopted for server and embedded applications.

ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0.

The development of the ECMAScript Language Specification started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.

That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.

The third edition of the Standard introduced powerful regular expressions, better string handling, new control statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minor changes in anticipation of future language growth. The third edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002 in June 2002.

After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the World Wide Web where it has become the programming language that is supported by essentially all web browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into the development of the sixth edition.

The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto interpretations of the language specification that have become common among browser implementations and added support for new features that had emerged since the publication of the third edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security. The fifth edition was adopted by the Ecma General Assembly of December 2009.

The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted by the Ecma General Assembly of June 2011.

Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for publication. However, this was preceded by significant experimentation and language enhancement design efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of the sixth edition is the culmination of a fifteen year effort. The goals for this edition included providing better support for large applications, library creation, and for use of ECMAScript as a compilation target for other languages. Some of its major enhancements included modules, class declarations, lexical block scoping, iterators and generators, promises for asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of built-ins was expanded to support additional data abstractions including maps, sets, and arrays of binary numeric values as well as additional support for Unicode supplemental characters in strings and regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides the foundation for regular, incremental language and library enhancements. The sixth edition was adopted by the General Assembly of June 2015.

ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence and open development process. A plain-text source document was built from the ECMAScript 2015 source document to serve as the base for further development entirely on GitHub. Over the year of this standard's development, hundreds of pull requests and issues were filed representing thousands of bug fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a new exponentiation operator and adds a new method to Array.prototype called includes.

ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics introduce a newmemory modelthat allows multi-agentprograms to communicate using atomic operations that ensure a well-defined execution order even on parallel CPUs. It also included new static methods on Object: Object.values, Object.entries, and Object.getOwnPropertyDescriptors.

ECMAScript 2018 introduced support for asynchronous iteration via the AsyncIterator protocol and async generators. It also included four new regular expression features: the dotAll flag, named capture groups, Unicode property escapes, and look-behind assertions. Lastly it included object rest and spread properties.

ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on Array.prototype for flattening arrays, Object.fromEntries for directly turning the return value of Object.entries into a new Object, and trimStart and trimEnd on String.prototype as better-named alternatives to the widely implemented but non-standard String.prototype.trimLeft and trimRight built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in string literals to align with JSON. Other updates included requiring that Array.prototype.sort be a stable sort, requiring that JSON.stringify return well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by requiring that it either return the corresponding original source text or a standard placeholder.

ECMAScript 2020, the 11th edition, introduces the matchAll method for Strings, to produce an iterator for all match objects generated by a global regular expression; import(), a syntax to asynchronously import Modules with a dynamic specifier; BigInt, a new number primitive for working with arbitrary precision integers; Promise.allSettled, a new Promise combinator that does not short-circuit; globalThis, a universal way to access the global this value; dedicated export * as ns from 'module' syntax for use within modules; increased standardization of for-in enumeration order; import.meta, ahost-populated object available in Modules that may contain contextual information about the Module; as well as adding two new syntax features to improve working with “nullish” values (null or undefined): nullish coalescing, a value selection operator; and optional chaining, a property access and function invocation operator that short-circuits if the value to access/invoke is nullish.

ECMAScript 2021, the 12th edition, introduces the replaceAll method for Strings; Promise.any, a Promise combinator that short-circuits when an input value is fulfilled; AggregateError, a new Error type to represent multiple errors at once; logical assignment operators (??=, &&=, ||=); WeakRef, for referring to a target object without preserving it from garbage collection, and FinalizationRegistry, to manage registration and unregistration of cleanup operations performed when target objects are garbage collected; separators for numeric literals (1_000); and Array.prototype.sort was made more precise, reducing the amount of cases that result in animplementation-definedsort order.

Dozens of individuals representing many organizations have made very significant contributions within Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed thousands of bug reports, performed implementation experiments, contributed test suites, and educated the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and acknowledge every person and organization who has contributed to this effort.

Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition

Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions

Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions

1 Scope

This Standard defines the ECMAScript 2022 general-purpose programming language.

2 Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects, properties, functions, and program syntax and semantics described in this specification.

A conforming implementation of ECMAScript must interpret source text input in conformance with the latest version of the Unicode Standard and ISO/IEC 10646.

A conforming implementation of ECMAScript that provides an application programming interface (API) that supports programs that need to adapt to the linguistic and cultural conventions used by different human languages and countries must implement the interface defined by the most recent edition of ECMA-402 that is compatible with this specification.

A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and functions beyond those described in this specification. In particular, a conforming implementation of ECMAScript may provide properties not described in this specification, and values for those properties, for objects that are described in this specification.

A conforming implementation of ECMAScript may support program and regular expression syntax not described in this specification. In particular, a conforming implementation of ECMAScript may support program syntax that makes use of any “future reserved words” noted in subclause12.6.2of this specification.

A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden Extension in subclause17.1.

A conforming implementation of ECMAScript must not redefine any facilities that are notimplementation-defined,implementation-approximated, orhost-defined.

A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A Normative Optional clause is denoted in this specification with the words "Normative Optional" in a coloured box, as shown below.

2.1 Example Normative Optional Clause Heading

Example clause contents.

A conforming implementation of ECMAScript must implement Legacy subclauses, unless they are also marked as Normative Optional. All of the language features and behaviours specified within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in existing applications prevents their removal from this specification. These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code.

2.2 Example Legacy Clause Heading

Example clause contents.

2.3 Example Legacy Normative Optional Clause Heading

Example clause contents.

3 Normative References

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO/IEC 10646 Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, and Amendment 4:2008, plus additional amendments and corrigenda, or successor

ECMA-402, ECMAScript 2015 Internationalization API Specification.
https://ecma-international.org/publications/standards/Ecma-402.htm

ECMA-404, The JSON Data Interchange Format.
https://ecma-international.org/publications/standards/Ecma-404.htm

4 Overview

This section contains a non-normative overview of the ECMAScript language.

ECMAScript is an object-oriented programming language for performing computations and manipulating computational objects within ahost environment. ECMAScript as defined here is not intended to be computationally self-sufficient; indeed, there are no provisions in this specification for input of external data or output of computed results. Instead, it is expected that the computational environment of an ECMAScript program will provide not only the objects and other facilities described in this specification but also certain environment-specific objects, whose description and behaviour are beyond the scope of this specification except to indicate that they may provide certain properties that can be accessed and certain functions that can be called from an ECMAScript program.

ECMAScript was originally designed to be used as a scripting language, but has become widely used as a general-purpose programming language. A scripting language is a programming language that is used to manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality is already available through a user interface, and the scripting language is a mechanism for exposing that functionality to program control. In this way, the existing system is said to provide ahost environmentof objects and facilities, which completes the capabilities of the scripting language. A scripting language is intended for use by both professional and non-professional programmers.

ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web pages in browsers and to perform server computation as part of a Web-based client-server architecture. ECMAScript is now used to provide core scripting capabilities for a variety ofhostenvironments. Therefore the core language is specified in this document apart from any particularhost environment.

ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.

4.1 Web Scripting

A web browser provides an ECMAScripthost environmentfor client-side computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies, and input/output. Further, thehost environmentprovides a means to attach scripting code to events such as change of focus, page and image loading, unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and the displayed page is a combination of user interface elements and fixed and computed text and images. The scripting code is reactive to user interaction, and there is no need for a main program.

A web server provides a differenthost environmentfor server-side computation including objects representing requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side scripting together, it is possible to distribute computation between the client and server while providing a customized user interface for a Web-based application.

Each Web browser and server that supports ECMAScript supplies its ownhost environment, completing the ECMAScript execution environment.

4.2 Hosts and Implementations

To aid integrating ECMAScript intohostenvironments, this specification defers the definition of certain facilities (e.g.,abstract operations), either in whole or in part, to a source outside of this specification. Editorially, this specification distinguishes the following kinds of deferrals.

An implementation is an external source that further defines facilities enumerated in AnnexDor those that are marked asimplementation-definedorimplementation-approximated. In informal use, an implementation refers to a concrete artefact, such as a particular web browser.

An implementation-defined facility is one that defers its definition to an external source without further qualification. This specification does not make any recommendations for particular behaviours, and conforming implementations are free to choose any behaviour within the constraints put forth by this specification.

An implementation-approximated facility is one that defers its definition to an external source while recommending an ideal behaviour. While conforming implementations are free to choose any behaviour within the constraints put forth by this specification, they are encouraged to strive to approximate the ideal. Some mathematical operations, such asMath.exp, areimplementation-approximated.

A host is an external source that further defines facilities listed in AnnexDbut does not further define otherimplementation-definedorimplementation-approximatedfacilities. In informal use, ahostrefers to the set of all implementations, such as the set of all web browsers, that interface with this specification in the same way via AnnexD. Ahostis often an external specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that arehost-definedare often further defined in external specifications.

A host hook is an abstract operation that is defined in whole or in part by an external source. Allhosthooks must be listed in AnnexD. Ahost hookmust conform to at least the following requirements:

A host-defined facility is one that defers its definition to an external source without further qualification and is listed in AnnexD. Implementations that are not hosts may also provide definitions forhost-definedfacilities.

A host environment is a particular choice of definition for allhost-definedfacilities. Ahost environmenttypically includes objects or functions which allow obtaining input and providing output ashost-definedproperties of theglobal object.

This specification follows the editorial convention of always using the most specific term. For example, if a facility ishost-defined, it should not be referred to asimplementation-defined.

Both hosts and implementations may interface with this specification via the language types, specification types,abstract operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.

4.3 ECMAScript Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This overview is not part of the standard proper.

ECMAScript is object-based: basic language andhostfacilities are provided by objects, and an ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of zero or more properties each with attributes that determine how each property can be used—for example, when the Writable attribute for a property is set tofalse, any attempt by executed ECMAScript code to assign a different value to the property fails. Properties are containers that hold other objects, primitive values, or functions. A primitive value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, BigInt, String, and Symbol; an object is a member of the built-in type Object; and a function is a callable object. A function that is associated with an object via a property is called a method.

ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. These built-in objects include theglobal object; objects that are fundamental to theruntime semanticsof the language including Object, Function, Boolean, Symbol, and various Error objects; objects that represent and manipulate numeric values including Math, Number, and Date; the text processing objects String and RegExp; objects that are indexed collections of values including Array and nine different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed collections including Map and Set objects; objects supporting structured data including the JSON object, ArrayBuffer, SharedArrayBuffer, and DataView; objects supporting control abstractions including generator functions and Promise objects; and reflection objects including Proxy and Reflect.

ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations, multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma operator.

Large ECMAScript programs are supported by modules which allow a program to be divided into multiple sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to be provided by other modules and which of its declarations are available for use by other modules.

ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are types associated with properties, and defined functions are not required to have their declarations appear textually before calls to them.

4.3.1 Objects

Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways including via a literal notation or via constructors which create objects and then execute code that initializes all or part of them by assigning initial values to their properties. Eachconstructoris a function that has a property named"prototype"that is used to implement prototype-based inheritance and shared properties. Objects are created by using constructors in new expressions; for example, new Date(2009, 11) creates a new Date object. Invoking aconstructorwithout using new has consequences that depend on theconstructor. For example, Date() produces a string representation of the current date and time rather than an object.

Every object created by aconstructorhas an implicit reference (called the object's prototype) to the value of itsconstructor's"prototype"property. Furthermore, a prototype may have a non-null implicit reference to its prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object, that reference is to the property of that name in the first object in the prototype chain that contains a property of that name. In other words, first the object mentioned directly is examined for such a property; if that object contains the named property, that is the property to which the reference refers; if that object does not contain the named property, the prototype for that object is examined next; and so on.

Figure 1: Object/Prototype Relationships
An image of lots of boxes and arrows.

In a class-based object-oriented language, in general, state is carried by instances, methods are carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carried by objects, while structure, behaviour, and state are all inherited.

All objects that do not directly contain a particular property that their prototype contains share that property and its value. Figure 1 illustrates this:

CF is aconstructor(and also an object). Five objects have been created by using new expressions: cf1, cf2, cf3, cf4, and cf5. Each of these objects contains properties named"q1"and"q2". The dashed lines represent the implicit prototype relationship; so, for example, cf3's prototype is CFp. Theconstructor, CF, has two properties itself, named"P1"and"P2", which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named"CFP1"in CFp is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp's implicit prototype chain that are not named"q1","q2", or"CFP1". Notice that there is no implicit prototype link between CF and CFp.

Unlike most class-based object languages, properties can be added to objects dynamically by assigning values to them. That is, constructors are not required to name or assign values to all or any of the constructed object's properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 by assigning a new value to the property in CFp.

Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like abstractions based upon a common pattern ofconstructorfunctions, prototype objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers to concisely define objects that conform to the same class-like abstraction pattern used by the built-in objects.

4.3.2 The Strict Variant of ECMAScript

The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict their usage of some features available in the language. They might do so in the interests of security, to avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variant of the language excludes some specific syntactic and semantic features of the regular ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies additional error conditions that must be reported by throwing error exceptions in situations that are not specified as errors by the non-strict form of the language.

The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level of individual ECMAScript source text units as described in11.2.2. Because strict mode is selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript semantics that must operate consistently across multiple source text units. A complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source text units. In this case, strict mode only applies when actually executing code that is defined within a strict mode source text unit.

In order to conform to this specification, an ECMAScript implementation must implement both the full unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this specification. In addition, an implementation must support the combination of unrestricted and strict mode source text units into a single composite program.

4.4 Terms and Definitions

For the purposes of this document, the following terms and definitions apply.

4.4.1 implementation-approximated

animplementation-approximatedfacility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this specification

4.4.2 implementation-defined

animplementation-definedfacility is defined in whole or in part by an external source to this specification

4.4.3 host-defined

same asimplementation-defined

Note

Editorially, see clause4.2.

4.4.4 type

set of data values as defined in clause6

4.4.5 primitive value

member of one of the types Undefined, Null, Boolean, Number, BigInt, Symbol, or String as defined in clause6

Note

A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.4.6 object

member of the type Object

Note

An object is a collection of properties and has a single prototype object. The prototype may be the null value.

4.4.7 constructor

function objectthat creates and initializes objects

Note

The value of aconstructor's"prototype"property is a prototype object that is used to implement inheritance and shared properties.

4.4.8 prototype

object that provides shared properties for other objects

Note

When aconstructorcreates an object, that object implicitly references theconstructor's"prototype"property for the purpose of resolving property references. Theconstructor's"prototype"property can be referenced by the program expression constructor.prototype, and properties added to an object's prototype are shared, through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified prototype by using the Object.create built-in function.

4.4.9 ordinary object

object that has the default behaviour for the essential internal methods that must be supported by all objects

4.4.10 exotic object

object that does not have the default behaviour for one or more of the essential internal methods

Note

Any object that is not anordinary objectis anexotic object.

4.4.11 standard object

object whose semantics are defined by this specification

4.4.12 built-in object

object specified and supplied by an ECMAScript implementation

Note

Standard built-in objects are defined in this specification. An ECMAScript implementation may specify and supply additional kinds of built-in objects. A built-inconstructor is a built-in object that is also aconstructor.

4.4.13 undefined value

primitive value used when a variable has not been assigned a value

4.4.14 Undefined type

type whose sole value is theundefinedvalue

4.4.15 null value

primitive value that represents the intentional absence of any object value

4.4.16 Null type

type whose sole value is thenullvalue

4.4.17 Boolean value

member of the Boolean type

Note

There are only two Boolean values,trueandfalse.

4.4.18 Boolean type

type consisting of the primitive valuestrueandfalse

4.4.19 Boolean object

member of the Object type that is an instance of the standard built-in Booleanconstructor

Note

A Boolean object is created by using the Booleanconstructorin a new expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose value is the Boolean value. A Boolean object can be coerced to a Boolean value.

4.4.20 String value

primitive value that is a finite ordered sequence of zero or more 16-bit unsignedintegervalues

Note

A String value is a member of the String type. Eachintegervalue in the sequence usually represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except that they must be 16-bit unsigned integers.

4.4.21 String type

set of all possible String values

4.4.22 String object

member of the Object type that is an instance of the standard built-in Stringconstructor

Note

A String object is created by using the Stringconstructorin a new expression, supplying a String value as an argument. The resulting object has an internal slot whose value is the String value. A String object can be coerced to a String value by calling the Stringconstructoras a function (22.1.1.1).

4.4.23 Number value

primitive value corresponding to a double-precision 64-bit binary formatIEEE 754-2019value

Note

ANumber valueis a member of the Number type and is a direct representation of a number.

4.4.24 Number type

set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and negative infinity

4.4.25 Number object

member of the Object type that is an instance of the standard built-in Numberconstructor

Note

A Number object is created by using the Numberconstructorin a new expression, supplying aNumber valueas an argument. The resulting object has an internal slot whose value is theNumber value. A Number object can be coerced to aNumber valueby calling the Numberconstructoras a function (21.1.1.1).

4.4.26 Infinity

Number valuethat is the positive infiniteNumber value

4.4.27 NaN

Number valuethat is anIEEE 754-2019“Not-a-Number” value

4.4.28 BigInt value

primitive value corresponding to an arbitrary-precisionintegervalue

4.4.29 BigInt type

set of all possible BigInt values

4.4.30 BigInt object

member of the Object type that is an instance of the standard built-in BigIntconstructor

4.4.31 Symbol value

primitive value that represents a unique, non-String Object property key

4.4.32 Symbol type

set of all possible Symbol values

4.4.33 Symbol object

member of the Object type that is an instance of the standard built-in Symbolconstructor

4.4.34 function

member of the Object type that may be invoked as a subroutine

Note

In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function's code may or may not be written in ECMAScript.

4.4.35 built-in function

built-in object that is a function

Note

Examples of built-in functions include parseInt and Math.exp. Ahostor implementation may provide additional built-in functions that are not described in this specification.

4.4.36 property

part of an object that associates a key (either a String value or a Symbol value) and a value

Note

Depending upon the form of the property the value may be represented either directly as a data value (a primitive value, an object, or afunction object) or indirectly by a pair of accessor functions.

4.4.37 method

function that is the value of a property

Note

When a function is called as a method of an object, the object is passed to the function as itsthisvalue.

4.4.38 built-in method

method that is a built-in function

Note

Standard built-in methods are defined in this specification. Ahostor implementation may provide additional built-in methods that are not described in this specification.

4.4.39 attribute

internal value that defines some characteristic of a property

4.4.40 own property

property that is directly contained by its object

4.4.41 inherited property

property of an object that is not an own property but is a property (either own or inherited) of the object's prototype

4.5 Organization of This Specification

The remainder of this specification is organized as follows:

Clause5defines the notational conventions used throughout the specification.

Clauses6through10define the execution environment within which ECMAScript programs operate.

Clauses11through17define the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all language features.

Clauses18through28define the ECMAScript standard library. They include the definitions of all of the standard objects that are available for use by ECMAScript programs as they execute.

Clause29describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.

5 Notational Conventions

5.1 Syntactic and Lexical Grammars

5.1.1 Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

A chain production is a production that has exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for which the nonterminal is the left-hand side.

5.1.2 The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause12. This grammar has as its terminal symbols Unicode code points that conform to the rules forSourceCharacterdefined in11.1. It defines a set of productions, starting from thegoal symbolInputElementDiv,InputElementTemplateTail, orInputElementRegExp, orInputElementRegExpOrTemplateTail, that describe how sequences of such code points are translated into a sequence of input elements.

Input elements other than white space and comments form the terminal symbols for the syntactic grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens, also become part of the stream of input elements and guide the process of automatic semicolon insertion (12.9). Simple white space and single-line comments are discarded and do not appear in the stream of input elements for the syntactic grammar. AMultiLineComment(that is, a comment of the form /**/ regardless of whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if aMultiLineCommentcontains one or more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of input elements for the syntactic grammar.

A RegExp grammar for ECMAScript is given in22.2.1. This grammar also has as its terminal symbols the code points as defined bySourceCharacter. It defines a set of productions, starting from thegoal symbolPattern, that describe how sequences of code points are translated into regular expression patterns.

Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating punctuation. The lexical and RegExp grammars share some productions.

5.1.3 The Numeric String Grammar

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of the lexical grammar having to do with numeric literals and has as its terminal symbolsSourceCharacter. This grammar appears in7.1.4.1.

Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.

5.1.4 The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses13through16. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, starting from two alternative goal symbolsScriptandModule, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.

When a stream of code points is to be parsed as an ECMAScriptScriptorModule, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (ScriptorModule), with no tokens left over.

When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node is a Parse Node. Each Parse Node is an instance of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse'sgoal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.

New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of identical source text. Parse Nodes are considered the same Parse Node if and only if they represent the same span of source text, are instances of the same grammar symbol, and resulted from the same parser invocation.

Note 1

Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:

let str = "1 + 1;";
eval(str);
eval(str);

Each call to eval converts the value of str into an ECMAScript source text and performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct even though each parse operates upon a source text that was derived from the same String value.

Note 2
Parse Nodes are specification artefacts, and implementations are not required to use an analogous data structure.

Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.

The syntactic grammar as presented in clauses13through16is not a complete account of which token sequences are accepted as a correct ECMAScriptScriptorModule. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain “awkward” places.

In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScriptScriptorModule. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive supplemental grammar is provided that further restricts the acceptable token sequences. Typically, anearly errorrule will then define an error condition if "P is not covering an N", where P is a Parse Node (an instance of the generalized production) and N is a nonterminal from the supplemental grammar. Here, the sequence of tokens originally matched by P is parsed again using N as thegoal symbol. (If N takes grammatical parameters, then they are set to the same values used when P was originally parsed.) An error occurs if the sequence of tokens cannot be parsed as a single instance of N, with no tokens left over. Subsequently, algorithms access the result of the parse using a phrase of the form "the N that is covered by P". This will always be a Parse Node (an instance of N, unique for a given P), since any parsing failure would have been detected by anearly errorrule.

5.1.5 Grammar Notation

Terminal symbols are shown in fixed width font, both in the productions of the grammars and throughout this specification whenever the text directly refers to such a terminal symbol. These are to appear in a script exactly as written. All terminal symbol code points specified in this way are to be understood as the appropriate Unicode code points from the Basic Latin range, as opposed to any similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be expressed by a \UnicodeEscapeSequence.

Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a “production”) is introduced by the name of the nonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic definition:

WhileStatement:while(Expression)Statement

states that the nonterminalWhileStatementrepresents the token while, followed by a left parenthesis token, followed by anExpression, followed by a right parenthesis token, followed by aStatement. The occurrences ofExpressionandStatementare themselves nonterminals. As another example, the syntactic definition:

ArgumentList:AssignmentExpressionArgumentList,AssignmentExpression

states that anArgumentListmay represent either a singleAssignmentExpressionor anArgumentList, followed by a comma, followed by anAssignmentExpression. This definition ofArgumentListis recursive, that is, it is defined in terms of itself. The result is that anArgumentListmay contain any positive number of arguments, separated by commas, where each argument expression is anAssignmentExpression. Such recursive definitions of nonterminals are common.

The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides, one that omits the optional element and one that includes it. This means that:

VariableDeclaration:BindingIdentifierInitializeropt

is a convenient abbreviation for:

VariableDeclaration:BindingIdentifierBindingIdentifierInitializer

and that:

ForStatement:for(LexicalDeclarationExpressionopt;Expressionopt)Statement

is a convenient abbreviation for:

ForStatement:for(LexicalDeclaration;Expressionopt)Statementfor(LexicalDeclarationExpression;Expressionopt)Statement

which in turn is an abbreviation for:

ForStatement:for(LexicalDeclaration;)Statementfor(LexicalDeclaration;Expression)Statementfor(LexicalDeclarationExpression;)Statementfor(LexicalDeclarationExpression;Expression)Statement

so, in this example, the nonterminalForStatementactually has four alternative right-hand sides.

A production may be parameterized by a subscripted annotation of the form “[parameters]”, which may appear as a suffix to the nonterminal symbol defined by the production. “parameters” may be either a single name or a comma separated list of names. A parameterized production is shorthand for a set of productions defining all combinations of the parameter names, preceded by an underscore, appended to the parameterized nonterminal symbol. This means that:

StatementList[Return]:ReturnStatementExpressionStatement

is a convenient abbreviation for:

StatementList:ReturnStatementExpressionStatementStatementList_Return:ReturnStatementExpressionStatement

and that:

StatementList[Return, In]:ReturnStatementExpressionStatement

is an abbreviation for:

StatementList:ReturnStatementExpressionStatementStatementList_Return:ReturnStatementExpressionStatementStatementList_In:ReturnStatementExpressionStatementStatementList_Return_In:ReturnStatementExpressionStatement

Multiple parameters produce a combinatory number of productions, not all of which are necessarily referenced in a complete grammar.

References to nonterminals on the right-hand side of a production can also be parameterized. For example:

StatementList:ReturnStatementExpressionStatement[+In]

is equivalent to saying:

StatementList:ReturnStatementExpressionStatement_In

and:

StatementList:ReturnStatementExpressionStatement[~In]

is equivalent to:

StatementList:ReturnStatementExpressionStatement

A nonterminal reference may have both a parameter list and an “opt” suffix. For example:

VariableDeclaration:BindingIdentifierInitializer[+In]opt

is an abbreviation for:

VariableDeclaration:BindingIdentifierBindingIdentifierInitializer_In

Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that parameter value dependent upon the occurrence of the parameter name on the reference to the current production's left-hand side symbol. For example:

VariableDeclaration[In]:BindingIdentifierInitializer[?In]

is an abbreviation for:

VariableDeclaration:BindingIdentifierInitializerVariableDeclaration_In:BindingIdentifierInitializer_In

If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was not used in referencing the production's nonterminal symbol. This means that:

StatementList[Return]:[+Return]ReturnStatementExpressionStatement

is an abbreviation for:

StatementList:ExpressionStatementStatementList_Return:ReturnStatementExpressionStatement

and that:

StatementList[Return]:[~Return]ReturnStatementExpressionStatement

is an abbreviation for:

StatementList:ReturnStatementExpressionStatementStatementList_Return:ExpressionStatement

When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal symbols on the following line or lines is an alternative definition. For example, the lexical grammar for ECMAScript contains the production:

NonZeroDigit::one of123456789

which is merely a convenient abbreviation for:

NonZeroDigit::123456789

If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's right-hand side contains no terminals or nonterminals.

If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates that the production may only be used if the token sequence seq is a prefix of the immediately following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a finite nonempty set of token sequences, indicates that the production may only be used if some element of set is a prefix of the immediately following token sequence. For convenience, the set can also be written as a nonterminal, in which case it represents the set of all token sequences to which that nonterminal could expand. It is considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.

These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production may only be used if seq is not a prefix of the immediately following input token sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no element of set is a prefix of the immediately following token sequence.

As an example, given the definitions:

DecimalDigit::one of0123456789DecimalDigits::DecimalDigitDecimalDigitsDecimalDigit

the definition:

LookaheadExample::n[lookahead ∉ {1,3,5,7,9}]DecimalDigitsDecimalDigit[lookahead ∉DecimalDigit]

matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit not followed by another decimal digit.

Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously identify the immediately following token sequence because determining later tokens requires knowing which lexicalgoal symbolto use at later positions. As such, when these are used in the syntactic grammar, it is considered an editorial error for a token sequence seq to appear in a lookahead restriction (including as part of a set of sequences) if the choices of lexical goal symbols to use could change whether or not seq would be a prefix of the resulting token sequence.

If the phrase “[noLineTerminatorhere]” appears in the right-hand side of a production of the syntactic grammar, it indicates that the production is a restricted production: it may not be used if aLineTerminatoroccurs in the input stream at the indicated position. For example, the production:

ThrowStatement:throw[noLineTerminatorhere]Expression;

indicates that the production may not be used if aLineTerminatoroccurs in the script between the throw token and theExpression.

Unless the presence of aLineTerminatoris forbidden by a restricted production, any number of occurrences ofLineTerminatormay appear between any two consecutive tokens in the stream of input elements without affecting the syntactic acceptability of the script.

When an alternative in a production of the lexical grammar or the numeric string grammar appears to be a multi-code point token, it represents the sequence of code points that would make up such a token.

The right-hand side of a production may specify that certain expansions are not permitted by using the phrase “but not” and then indicating the expansions to be excluded. For example, the production:

Identifier::IdentifierNamebut notReservedWord

means that the nonterminalIdentifiermay be replaced by any sequence of code points that could replaceIdentifierNameprovided that the same sequence of code points could not replaceReservedWord.

Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it would be impractical to list all the alternatives:

SourceCharacter::any Unicode code point

5.2 Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms available to implement a given feature.

Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may be used within the algorithm steps to reference the argument passed in that position. Optional parameters are denoted with surrounding brackets ([ , name ]) and are no different from required parameters within algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures all of the arguments provided following the required and optional parameters into aList. If there are no such additional arguments, thatListis empty.

Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be further divided into indented substeps. Outline numbering conventions are used to identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second level of substeps labelled with lower case roman numerals. If more than three levels are required these rules repeat with the fourth level using numeric labels. For example:

  1. Top-level step
    1. Substep.
    2. Substep.
      1. Subsubstep.
        1. Subsubsubstep
          1. Subsubsubsubstep
            1. Subsubsubsubsubstep

A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is the negation of the preceding “if” predicate step at the same level.

A step may specify the iterative application of its substeps.

A step that begins with “Assert:” asserts an invariant condition of its algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be checked by an implementation. They are used simply to clarify algorithms.

Algorithm steps may declare named aliases for any value using the form “Let x be someValue”. These aliases are reference-like in that both x and someValue refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x be a copy of someValue” creates a shallow copy of someValue.

Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior to the alias's declaration. Aliases may be modified using the form “Set x to someOtherValue”.

5.2.1 Abstract Operations

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that they may be referenced by name from within other algorithms. Abstract operations are typically referenced using a functional application style such as OperationName(arg1, arg2). Some abstract operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such method-like abstract operations are typically referenced using a method application style such as someValue.OperationName(arg1, arg2).

5.2.2 Syntax-Directed Operations

A syntax-directed operation is a named operation whose definition consists of algorithms, each of which is associated with one or more productions from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production, it may reference the terminal and nonterminal symbols of the production alternative as if they were parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative definition that is matched when parsing the source text. The source text matched by a grammar production is the portion of the source text that starts at the beginning of the first terminal that participated in the match and ends at the end of the last terminal that participated in the match.

When an algorithm is associated with a production alternative, the alternative is typically shown without any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the alternative and have no effect on the associated semantics for the alternative.

Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the conventions on steps1,3, and4in the following algorithm:

  1. Let status be SyntaxDirectedOperation ofSomeNonTerminal.
  2. Let someParseNode be the parse of some source text.
  3. Perform SyntaxDirectedOperation of someParseNode.
  4. Perform SyntaxDirectedOperation of someParseNode passing"value"as the argument.

Unless explicitly specified otherwise, all chain productions have an implicit definition for every operation that might be applied to that production's left-hand side nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any, to thechain production's sole right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step of the form: “Return the result of evaluatingBlock” and that there is a production:

Block:{StatementList}

but the Evaluation operation does not associate an algorithm with that production. In that case, the Evaluation operation implicitly includes an association of the form:

Runtime Semantics: Evaluation

Block:{StatementList}
  1. Return the result of evaluatingStatementList.

5.2.3 Runtime Semantics

Algorithms which specify semantics that must be called at runtime are called runtime semantics. Runtime semantics are defined byabstract operationsor syntax-directed operations. Such algorithms always return a completion record.

5.2.3.1 Implicit Completion Values

The algorithms of this specification often implicitly returnCompletionRecords whose [[Type]] isnormal. Unless it is otherwise obvious from the context, an algorithm statement that returns a value that is not aCompletion Record, such as:

  1. Return"Infinity".

means the same thing as:

  1. ReturnNormalCompletion("Infinity").

However, if the value expression of a “return” statement is aCompletion Recordconstruction literal, the resultingCompletion Recordis returned. If the value expression is a call to an abstract operation, the “return” statement simply returns theCompletion Recordproduced by the abstract operation.

The abstract operationCompletion(completionRecord) is used to emphasize that a previously computedCompletion Recordis being returned. TheCompletionabstract operation takes a single argument, completionRecord, and performs the following steps:

  1. Assert: completionRecord is aCompletion Record.
  2. Return completionRecord as theCompletion Recordof this abstract operation.

A “return” statement without a value in an algorithm step means the same thing as:

  1. ReturnNormalCompletion(undefined).

Any reference to aCompletion Recordvalue that is in a context that does not explicitly require a completeCompletion Recordvalue is equivalent to an explicit reference to the [[Value]] field of theCompletion Recordvalue unless theCompletion Recordis anabrupt completion.

5.2.3.2 Throw an Exception

Algorithms steps that say to throw an exception, such as

  1. Throw aTypeErrorexception.

mean the same things as:

  1. ReturnThrowCompletion(a newly createdTypeErrorobject).

5.2.3.3 ReturnIfAbrupt

Algorithms steps that say or are otherwise equivalent to:

  1. ReturnIfAbrupt(argument).

mean the same thing as:

  1. If argument is anabrupt completion, return argument.
  2. Else if argument is aCompletion Record, set argument to argument.[[Value]].

Algorithms steps that say or are otherwise equivalent to:

  1. ReturnIfAbrupt(AbstractOperation()).

mean the same thing as:

  1. Let hygienicTemp be AbstractOperation().
  2. If hygienicTemp is anabrupt completion, return hygienicTemp.
  3. Else if hygienicTemp is aCompletion Record, set hygienicTemp to hygienicTemp.[[Value]].

Where hygienicTemp is ephemeral and visible only in the steps pertaining to ReturnIfAbrupt.

Algorithms steps that say or are otherwise equivalent to:

  1. Let result be AbstractOperation(ReturnIfAbrupt(argument)).

mean the same thing as:

  1. If argument is anabrupt completion, return argument.
  2. If argument is aCompletion Record, set argument to argument.[[Value]].
  3. Let result be AbstractOperation(argument).

5.2.3.4 ReturnIfAbrupt Shorthands

Invocations ofabstract operationsand syntax-directed operations that are prefixed by ? indicate thatReturnIfAbruptshould be applied to the resultingCompletion Record. For example, the step:

  1. ? OperationName().

is equivalent to the following step:

  1. ReturnIfAbrupt(OperationName()).

Similarly, for method application style, the step:

  1. ? someValue.OperationName().

is equivalent to:

  1. ReturnIfAbrupt(someValue.OperationName()).

Similarly, prefix ! is used to indicate that the following invocation of an abstract or syntax-directed operation will never return anabrupt completionand that the resultingCompletion Record's [[Value]] field should be used in place of the return value of the operation. For example, the step:

  1. Let val be ! OperationName().

is equivalent to the following steps:

  1. Let val be OperationName().
  2. Assert: val is never anabrupt completion.
  3. If val is aCompletion Record, set val to val.[[Value]].

Syntax-directed operations forruntime semanticsmake use of this shorthand by placing ! or ? before the invocation of the operation:

  1. Perform ! SyntaxDirectedOperation ofNonTerminal.

5.2.4 Static Semantics

Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream of input elements form a valid ECMAScriptScriptorModulethat may be evaluated. In some situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions or prose requirements. Such rules are always associated with a production of a grammar and are called the static semantics of the production.

Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules are associated with grammar productions and a production that has multiple alternative definitions will typically have for each alternative a distinct algorithm for each applicable named static semantic rule.

A special kind of static semantic rule is an Early Error Rule.Early errorrules defineearly errorconditions (see clause17) that are associated with specific grammar productions. Evaluation of mostearly errorrules are not explicitly invoked within the algorithms of this specification. A conforming implementation must, prior to the first evaluation of aScriptorModule, validate all of theearly errorrules of the productions used to parse thatScriptorModule. If any of theearly errorrules are violated theScriptorModuleis invalid and cannot be evaluated.

5.2.5 Mathematical Operations

This specification makes reference to these kinds of numeric values:

  • Mathematical values: Arbitrary real numbers, used as the default numeric type.
  • Extended mathematical values: Mathematical values together with +∞ and -∞.
  • Numbers:IEEE 754-2019double-precision floating point values.
  • BigInts: ECMAScript values representing arbitrary integers in a one-to-one correspondence.

In the language of this specification, numerical values are distinguished among different numeric kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript refers to BigInts. Numeric values without a subscript suffix refer to mathematical values.

Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the operands. When applied to mathematical values, the operators refer to the usual mathematical operations. When applied to Numbers, the operators refer to the relevant operations withinIEEE 754-2019. When applied to BigInts, the operators refer to the usual mathematical operations applied to themathematical valueof the BigInt.

In general, when this specification refers to a numerical value, such as in the phrase, "the length of y" or "theintegerrepresented by the four hexadecimal digits ...", without explicitly specifying a numeric kind, the phrase refers to amathematical value. Phrases which refer to a Number or a BigInt value are explicitly annotated as such; for example, "theNumber valuefor the number of code points in …" or "the BigInt value for …".

Numeric operators applied to mixed-type operands (such as a Number and amathematical value) are not defined and should be considered an editorial error in this specification.

This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.

When the term integer is used in this specification, it refers to amathematical valuewhich is in the set of integers, unless otherwise stated. When the term integral Number is used in this specification, it refers to aNumber valuewhosemathematical valueis in the set of integers.

Conversions between mathematical values and Numbers or BigInts are always explicit in this document. A conversion from amathematical valueorextended mathematical valuex to a Number is denoted as "theNumber valuefor x" or 𝔽(x), and is defined in6.1.6.1. A conversion from anintegerx to a BigInt is denoted as "the BigInt value for x" or (x). A conversion from a Number or BigInt x to amathematical valueis denoted as "the mathematical value of x", or (x). Themathematical valueof+0𝔽 and-0𝔽 is themathematical value0. Themathematical valueof non-finite values is not defined. The extended mathematical value of x is themathematical valueof x for finite values, and is +∞ and -∞ for+∞𝔽 and-∞𝔽 respectively; it is not defined forNaN.

The mathematical functionabs(x)produces the absolute value of x, which is-xif x < 0 and otherwise is x itself.

The mathematical functionmin(x1, x2, … , xN)produces the mathematically smallest ofx1throughxN. The mathematical functionmax(x1, x2, ..., xN)produces the mathematically largest ofx1throughxN. The domain and range of these mathematical functions are the extended mathematical values.

The notation “x modulo y” (y must be finite and non-zero) computes a value k of the same sign as y (or zero) such thatabs(k) <abs(y) and x - k = q × yfor someintegerq.

The phrase "the result of clamping x between lower and upper" (where x is anextended mathematical valueand lower and upper are mathematical values such that lowerupper) produces lower if x < lower, produces upper if x > upper, and otherwise produces x.

The mathematical functionfloor(x)produces the largestinteger(closest to +∞) that is not larger than x.

Mathematical functions min, max,abs, andfloorare not defined for Numbers and BigInts, and any usage of those methods that have non-mathematical valuearguments would be an editorial error in this specification.

Note

floor(x) = x - (xmodulo1).

5.2.6 Value Notation

In this specification, ECMAScript language values are displayed inbold. Examples includenull,true, or"hello". These are distinguished from longer ECMAScript code sequences such as Function.prototype.apply or let n = 42;.

Values which are internal to the specification and not directly observable from ECMAScript code are indicated with asans-seriftypeface. For instance, aCompletion Record's [[Type]] field takes on values likenormal,return, orthrow.

6 ECMAScript Data Types and Values

Algorithms within this specification manipulate values each of which has an associated type. The possible value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types and specification types.

Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the ECMAScript language and specification types defined in this clause. When the term “empty” is used as if it was naming a value, it is equivalent to saying “no value of any type”.

6.1 ECMAScript Language Types

An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is characterized by an ECMAScript language type.

6.1.1 The Undefined Type

The Undefined type has exactly one value, calledundefined. Any variable that has not been assigned a value has the valueundefined.

6.1.2 The Null Type

The Null type has exactly one value, callednull.

6.1.3 The Boolean Type

The Boolean type represents a logical entity having two values, calledtrueandfalse.

6.1.4 The String Type

The String type is the set of all ordered sequences of zero or more 16-bit unsignedintegervalues (“elements”) up to a maximum length of 253 - 1 elements. The String type is generally used to represent textual data in a running ECMAScript program, in which case each element in the String is treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence. These positions are indexed with non-negative integers. The first element (if any) is at index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.

ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not restrict the value of or relationships between these code units, so operations that further interpret String contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to every code unit with a numeric value in the inclusive range 0xD800 to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit with a numeric value in the inclusive range 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a low-surrogate code unit) using the following rules:

The function String.prototype.normalize (see22.1.3.13) can be used to explicitly normalize a String value. String.prototype.localeCompare (see22.1.3.10) internally normalizes String values, but no other operations implicitly normalize the strings upon which they operate. Only operations that are explicitly specified to be language or locale sensitive produce language-sensitive results.

Note

The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. If ECMAScript source text is in Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain any Unicode escape sequences.

In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code units is the concatenation of the code units (in order) of each of the arguments (in order).

The phrase "the substring of S from inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the String value consisting of the consecutive code units of S beginning at index inclusiveStart and ending immediately before index exclusiveEnd (which is the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of S is used as the value of exclusiveEnd.

6.1.4.1 StringIndexOf ( string, searchValue, fromIndex )

The abstract operation StringIndexOf takes arguments string (a String), searchValue (a String), and fromIndex (a non-negativeinteger). It performs the following steps when called:

  1. Assert:Type(string) is String.
  2. Assert:Type(searchValue) is String.
  3. Assert: fromIndex is a non-negativeinteger.
  4. Let len be the length of string.
  5. If searchValue is the empty String and fromIndexlen, return fromIndex.
  6. Let searchLen be the length of searchValue.
  7. For eachintegeri starting with fromIndex such that ilen - searchLen, in ascending order, do
    1. Let candidate be thesubstringof string from i to i + searchLen.
    2. If candidate is the same sequence of code units as searchValue, return i.
  8. Return -1.
Note 1

If searchValue is the empty String and fromIndex is less than or equal to the length of string, this algorithm returns fromIndex. The empty String is effectively found at every position within a string, including after the last code unit.

Note 2

This algorithm always returns -1 if fromIndex > the length of string.

6.1.5 The Symbol Type

The Symbol type is the set of all non-String values that may be used as the key of an Object property (6.1.7).

Each possible Symbol value is unique and immutable.

Each Symbol value immutably holds an associated value called [[Description]] that is eitherundefinedor a String value.

6.1.5.1 Well-Known Symbols

Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this specification. They are typically used as the keys of properties whose values serve as extension points of a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all realms (9.3).

Within this specification a well-known symbol is referred to by using a notation of the form @@name, where “name” is one of the values listed inTable 1.

Table 1: Well-known Symbols
Specification Name[[Description]]Value and Purpose
@@asyncIterator"Symbol.asyncIterator"A method that returns the default AsyncIterator for an object. Called by the semantics of the for-await-of statement.
@@hasInstance"Symbol.hasInstance"A method that determines if aconstructorobject recognizes an object as one of theconstructor's instances. Called by the semantics of the instanceof operator.
@@isConcatSpreadable"Symbol.isConcatSpreadable"A Boolean valued property that if true indicates that an object should be flattened to its array elements byArray.prototype.concat.
@@iterator"Symbol.iterator"A method that returns the default Iterator for an object. Called by the semantics of the for-of statement.
@@match"Symbol.match"A regular expression method that matches the regular expression against a string. Called by theString.prototype.matchmethod.
@@matchAll"Symbol.matchAll"A regular expression method that returns an iterator, that yields matches of the regular expression against a string. Called by theString.prototype.matchAllmethod.
@@replace"Symbol.replace"A regular expression method that replaces matched substrings of a string. Called by theString.prototype.replacemethod.
@@search"Symbol.search"A regular expression method that returns the index within a string that matches the regular expression. Called by theString.prototype.searchmethod.
@@species"Symbol.species"A function valued property that is theconstructorfunction that is used to create derived objects.
@@split"Symbol.split"A regular expression method that splits a string at the indices that match the regular expression. Called by theString.prototype.splitmethod.
@@toPrimitive"Symbol.toPrimitive"A method that converts an object to a corresponding primitive value. Called by theToPrimitiveabstract operation.
@@toStringTag"Symbol.toStringTag"A String valued property that is used in the creation of the default string description of an object. Accessed by the built-in methodObject.prototype.toString.
@@unscopables"Symbol.unscopables"An object valued property whose own and inherited property names are property names that are excluded from the with environment bindings of the associated object.

6.1.6 Numeric Types

ECMAScript has two built-in numeric types: Number and BigInt. In this specification, every numeric type T contains a multiplicative identity value denoted T::unit. The specification types also have the followingabstract operations, likewise denoted T::op for a given operation with specification name op. All argument types are T. The "Result" column shows the return type, along with an indication if it is possible for some invocations of the operation to return anabrupt completion.

Table 2: Numeric Type Operations
Invocation SynopsisExample sourceInvoked by the Evaluation semantics of ...Result
T::unaryMinus(x)-xUnary - OperatorT
T::bitwiseNOT(x)~xBitwise NOT Operator ( ~ )T
T::exponentiate(x, y)x ** yExponentiation OperatorandMath.pow ( base, exponent )T, may throwRangeError
T::multiply(x, y)x * yMultiplicative OperatorsT
T::divide(x, y)x / yMultiplicative OperatorsT, may throwRangeError
T::remainder(x, y)x % yMultiplicative OperatorsT, may throwRangeError
T::add(x, y)x ++
++ x
x + y
Postfix Increment Operator,Prefix Increment Operator, andThe Addition Operator ( + )T
T::subtract(x, y)x --
-- x
x - y
Postfix Decrement Operator,Prefix Decrement Operator, andThe Subtraction Operator ( - )T
T::leftShift(x, y)x << yThe Left Shift Operator ( << )T
T::signedRightShift(x, y)x >> yThe Signed Right Shift Operator ( >> )T
T::unsignedRightShift(x, y)x >>> yThe Unsigned Right Shift Operator ( >>> )T, may throwTypeError
T::lessThan(x, y)x < y
x > y
x <= y
x >= y
Relational Operators, viaIsLessThan ( x, y, LeftFirst )Boolean orundefined(for unordered inputs)
T::equal(x, y)x == y
x != y
x === y
x !== y
Equality Operators, viaIsStrictlyEqual ( x, y )Boolean
T::sameValue(x, y)Object internal methods, viaSameValue ( x, y ), to test exact value equalityBoolean
T::sameValueZero(x, y)Array, Map, and Set methods, viaSameValueZero ( x, y ), to test value equality ignoring differences among members of the zero cohort (i.e.,-0𝔽 and+0𝔽)Boolean
T::bitwiseAND(x, y)x & yBinary Bitwise OperatorsT
T::bitwiseXOR(x, y)x ^ yBinary Bitwise OperatorsT
T::bitwiseOR(x, y)x | yBinary Bitwise OperatorsT
T::toString(x)String(x)Many expressions and built-in functions, viaToString ( argument )String

The T::unit value and T::op operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Otherabstract operationsare defined throughout this specification.

Because the numeric types are in general not convertible without loss of precision or truncation, the ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call Number and BigInt functions to convert among types when calling a function which requires another type.

Note

The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric conversions that could lose precision or truncate. These legacy implicit conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize opportunity for programmer error, and to leave open the option of generalized value types in a future edition.

6.1.6.1 The Number Type

The Number type has exactly 18,437,736,874,454,810,627 (that is,264 - 253 + 3) values, representing the double-precision 64-bit formatIEEE 754-2019values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is,253 - 2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a single specialNaNvalue. (Note that theNaNvalue is produced by the program expression NaN.) In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour isimplementation-defined; to ECMAScript code, allNaNvalues are indistinguishable from each other.

Note

The bit pattern that might be observed in an ArrayBuffer (see25.1) or a SharedArrayBuffer (see25.2) after aNumber valuehas been stored into it is not necessarily the same as the internal representation of thatNumber valueused by the ECMAScript implementation.

There are two other special values, calledpositive Infinityandnegative Infinity. For brevity, these values are also referred to for expository purposes by the symbols+∞𝔽 and-∞𝔽, respectively. (Note that these two infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and -Infinity.)

The other 18,437,736,874,454,810,624 (that is,264 - 253) values are called the finite numbers. Half of these are positive numbers and half are negative numbers; for every finite positiveNumber valuethere is a corresponding negative value having the same magnitude.

Note that there is both apositive zeroand anegative zero. For brevity, these values are also referred to for expository purposes by the symbols+0𝔽 and-0𝔽, respectively. (Note that these two different zero Number values are produced by the program expressions +0 (or simply 0) and -0.)

The 18,437,736,874,454,810,622 (that is,264 - 253 - 2) finite non-zero values are of two kinds:

18,428,729,675,200,069,632 (that is,264 - 254) of them are normalized, having the form

s × m × 2e

where s is 1 or -1, m is anintegersuch that 252m < 253, and e is anintegersuch that -1074 ≤ e ≤ 971.

The remaining 9,007,199,254,740,990 (that is,253 - 2) values are denormalized, having the form

s × m × 2e

where s is 1 or -1, m is anintegersuch that 0 < m < 252, and e is -1074.

Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in the Number type. Theinteger0 has two representations in the Number type:+0𝔽 and-0𝔽.

A finite number has an odd significand if it is non-zero and theintegerm used to express it (in one of the two forms shown above) is odd. Otherwise, it has an even significand.

In this specification, the phrase “the Number value for x” where x represents an exact real mathematical quantity (which might even be an irrational number such as π) means aNumber valuechosen in the following manner. Consider the set of all finite values of the Number type, with-0𝔽 removed and with two additional values added to it that are not representable in the Number type, namely 21024 (which is+1 × 253 × 2971) and-21024(which is-1 × 253 × 2971). Choose the member of this set that is closest in value to x. If two values of the set are equally close, then the one with an even significand is chosen; for this purpose, the two extra values 21024 and-21024are considered to have even significands. Finally, if 21024 was chosen, replace it with+∞𝔽; if-21024was chosen, replace it with-∞𝔽; if+0𝔽 was chosen, replace it with-0𝔽 if and only if x < 0; any other chosen value is used unchanged. The result is theNumber valuefor x. (This procedure corresponds exactly to the behaviour of theIEEE 754-2019roundTiesToEven mode.)

TheNumber valuefor +∞ is+∞𝔽, and theNumber valuefor -∞ is-∞𝔽.

Some ECMAScript operators deal only with integers in specific ranges such as-231through231 - 1, inclusive, or in the range 0 through216 - 1, inclusive. These operators accept any value of the Number type but first convert each such value to anintegervalue in the expected range. See the descriptions of the numeric conversion operations in7.1.

The Number::unit value is1𝔽.

6.1.6.1.1 Number::unaryMinus ( x )

The abstract operation Number::unaryMinus takes argument x (a Number). It performs the following steps when called:

  1. If x isNaN, returnNaN.
  2. Return the result of negating x; that is, compute a Number with the same magnitude but opposite sign.

6.1.6.1.2 Number::bitwiseNOT ( x )

The abstract operation Number::bitwiseNOT takes argument x (a Number). It performs the following steps when called:

  1. Let oldValue be ! ToInt32(x).
  2. Return the result of applying bitwise complement to oldValue. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.3 Number::exponentiate ( base, exponent )

The abstract operation Number::exponentiate takes arguments base (a Number) and exponent (a Number). It returns animplementation-approximatedvalue representing the result of raising base to the exponent power. It performs the following steps when called:

  1. If exponent isNaN, returnNaN.
  2. If exponent is+0𝔽 or exponent is-0𝔽, return1𝔽.
  3. If base isNaN, returnNaN.
  4. If base is+∞𝔽, then
    1. If exponent >+0𝔽, return+∞𝔽. Otherwise, return+0𝔽.
  5. If base is-∞𝔽, then
    1. If exponent >+0𝔽, then
      1. If exponent is an oddintegral Number, return-∞𝔽. Otherwise, return+∞𝔽.
    2. Else,
      1. If exponent is an oddintegral Number, return-0𝔽. Otherwise, return+0𝔽.
  6. If base is+0𝔽, then
    1. If exponent >+0𝔽, return+0𝔽. Otherwise, return+∞𝔽.
  7. If base is-0𝔽, then
    1. If exponent >+0𝔽, then
      1. If exponent is an oddintegral Number, return-0𝔽. Otherwise, return+0𝔽.
    2. Else,
      1. If exponent is an oddintegral Number, return-∞𝔽. Otherwise, return+∞𝔽.
  8. Assert: base is finite and is neither+0𝔽 nor-0𝔽.
  9. If exponent is+∞𝔽, then
    1. Ifabs((base)) > 1, return+∞𝔽.
    2. Ifabs((base)) is 1, returnNaN.
    3. Ifabs((base)) < 1, return+0𝔽.
  10. If exponent is-∞𝔽, then
    1. Ifabs((base)) > 1, return+0𝔽.
    2. Ifabs((base)) is 1, returnNaN.
    3. Ifabs((base)) < 1, return+∞𝔽.
  11. Assert: exponent is finite and is neither+0𝔽 nor-0𝔽.
  12. If base <+0𝔽 and exponent is not anintegral Number, returnNaN.
  13. Return animplementation-approximatedvalue representing the result of raising(base) to the(exponent) power.
Note

The result of base ** exponent when base is1𝔽 or-1𝔽 and exponent is+∞𝔽 or-∞𝔽, or when base is1𝔽 and exponent isNaN, differs fromIEEE 754-2019. The first edition of ECMAScript specified a result ofNaNfor this operation, whereas later versions ofIEEE 754-2019specified1𝔽. The historical ECMAScript behaviour is preserved for compatibility reasons.

6.1.6.1.4 Number::multiply ( x, y )

The abstract operation Number::multiply takes arguments x (a Number) and y (a Number). It performs multiplication according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the product of x and y. It performs the following steps when called:

  1. If x isNaNor y isNaN, returnNaN.
  2. If x is+∞𝔽 or x is-∞𝔽, then
    1. If y is+0𝔽 or y is-0𝔽, returnNaN.
    2. If y >+0𝔽, return x.
    3. Return -x.
  3. If y is+∞𝔽 or y is-∞𝔽, then
    1. If x is+0𝔽 or x is-0𝔽, returnNaN.
    2. If x >+0𝔽, return y.
    3. Return -y.
  4. Return𝔽((x) ×(y)).
Note

Finite-precision multiplication is commutative, but not always associative.

6.1.6.1.5 Number::divide ( x, y )

The abstract operation Number::divide takes arguments x (a Number) and y (a Number). It performs division according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the quotient of x and y where x is the dividend and y is the divisor. It performs the following steps when called:

  1. If x isNaNor y isNaN, returnNaN.
  2. If x is+∞𝔽 or x is-∞𝔽, then
    1. If y is+∞𝔽 or y is-∞𝔽, returnNaN.
    2. If y is+0𝔽 or y >+0𝔽, return x.
    3. Return -x.
  3. If y is+∞𝔽, then
    1. If x is+0𝔽 or x >+0𝔽, return+0𝔽. Otherwise, return-0𝔽.
  4. If y is-∞𝔽, then
    1. If x is+0𝔽 or x >+0𝔽, return-0𝔽. Otherwise, return+0𝔽.
  5. If x is+0𝔽 or x is-0𝔽, then
    1. If y is+0𝔽 or y is-0𝔽, returnNaN.
    2. If y >+0𝔽, return x.
    3. Return -x.
  6. If y is+0𝔽, then
    1. If x >+0𝔽, return+∞𝔽. Otherwise, return-∞𝔽.
  7. If y is-0𝔽, then
    1. If x >+0𝔽, return-∞𝔽. Otherwise, return+∞𝔽.
  8. Return𝔽((x) /(y)).

6.1.6.1.6 Number::remainder ( n, d )

The abstract operation Number::remainder takes arguments n (a Number) and d (a Number). It yields the remainder from an implied division of its operands where n is the dividend and d is the divisor. It performs the following steps when called:

  1. If n isNaNor d isNaN, returnNaN.
  2. If n is+∞𝔽 or n is-∞𝔽, returnNaN.
  3. If d is+∞𝔽 or d is-∞𝔽, return n.
  4. If d is+0𝔽 or d is-0𝔽, returnNaN.
  5. If n is+0𝔽 or n is-0𝔽, return n.
  6. Assert: n and d are finite and non-zero.
  7. Let r be(n) - ((d) × q) where q is anintegerthat is negative if and only if n and d have opposite sign, and whose magnitude is as large as possible without exceeding the magnitude of(n) /(d).
  8. Return𝔽(r).
Note 1

In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floating-point operands.

Note 2
The result of a floating-point remainder operation as computed by the % operator is not the same as the “remainder” operation defined byIEEE 754-2019. TheIEEE 754-2019“remainder” operation computes the remainder from a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usualintegerremainder operator. Instead the ECMAScript language defines % on floating-point operations to behave in a manner analogous to that of the Javaintegerremainder operator; this may be compared with the C library function fmod.

6.1.6.1.7 Number::add ( x, y )

The abstract operation Number::add takes arguments x (a Number) and y (a Number). It performs addition according to the rules ofIEEE 754-2019binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps when called:

  1. If x isNaNor y isNaN, returnNaN.
  2. If x is+∞𝔽 and y is-∞𝔽, returnNaN.
  3. If x is-∞𝔽 and y is+∞𝔽, returnNaN.
  4. If x is+∞𝔽 or x is-∞𝔽, return x.
  5. If y is+∞𝔽 or y is-∞𝔽, return y.
  6. Assert: x and y are both finite.
  7. If x is-0𝔽 and y is-0𝔽, return-0𝔽.
  8. Return𝔽((x) +(y)).
Note

Finite-precision addition is commutative, but not always associative.

6.1.6.1.8 Number::subtract ( x, y )

The abstract operation Number::subtract takes arguments x (a Number) and y (a Number). It performs subtraction, producing the difference of its operands; x is the minuend and y is the subtrahend. It performs the following steps when called:

  1. ReturnNumber::add(x,Number::unaryMinus(y)).
Note

It is always the case that x - y produces the same result as x + (-y).

6.1.6.1.9 Number::leftShift ( x, y )

The abstract operation Number::leftShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. Let lnum be ! ToInt32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be(rnum)modulo32.
  4. Return the result of left shifting lnum by shiftCount bits. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.10 Number::signedRightShift ( x, y )

The abstract operation Number::signedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. Let lnum be ! ToInt32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be(rnum)modulo32.
  4. Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant bit is propagated. Themathematical valueof the result is exactly representable as a 32-bit two's complement bit string.

6.1.6.1.11 Number::unsignedRightShift ( x, y )

The abstract operation Number::unsignedRightShift takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. Let lnum be ! ToUint32(x).
  2. Let rnum be ! ToUint32(y).
  3. Let shiftCount be(rnum)modulo32.
  4. Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filled with zero. Themathematical valueof the result is exactly representable as a 32-bit unsigned bit string.

6.1.6.1.12 Number::lessThan ( x, y )

The abstract operation Number::lessThan takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. If x isNaN, returnundefined.
  2. If y isNaN, returnundefined.
  3. If x and y are the sameNumber value, returnfalse.
  4. If x is+0𝔽 and y is-0𝔽, returnfalse.
  5. If x is-0𝔽 and y is+0𝔽, returnfalse.
  6. If x is+∞𝔽, returnfalse.
  7. If y is+∞𝔽, returntrue.
  8. If y is-∞𝔽, returnfalse.
  9. If x is-∞𝔽, returntrue.
  10. Assert: x and y are finite and non-zero.
  11. If(x) <(y), returntrue. Otherwise, returnfalse.

6.1.6.1.13 Number::equal ( x, y )

The abstract operation Number::equal takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. If x isNaN, returnfalse.
  2. If y isNaN, returnfalse.
  3. If x is the sameNumber valueas y, returntrue.
  4. If x is+0𝔽 and y is-0𝔽, returntrue.
  5. If x is-0𝔽 and y is+0𝔽, returntrue.
  6. Returnfalse.

6.1.6.1.14 Number::sameValue ( x, y )

The abstract operation Number::sameValue takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. If x isNaNand y isNaN, returntrue.
  2. If x is+0𝔽 and y is-0𝔽, returnfalse.
  3. If x is-0𝔽 and y is+0𝔽, returnfalse.
  4. If x is the sameNumber valueas y, returntrue.
  5. Returnfalse.

6.1.6.1.15 Number::sameValueZero ( x, y )

The abstract operation Number::sameValueZero takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. If x isNaNand y isNaN, returntrue.
  2. If x is+0𝔽 and y is-0𝔽, returntrue.
  3. If x is-0𝔽 and y is+0𝔽, returntrue.
  4. If x is the sameNumber valueas y, returntrue.
  5. Returnfalse.

6.1.6.1.16 NumberBitwiseOp ( op, x, y )

The abstract operation NumberBitwiseOp takes arguments op (a sequence of Unicode code points), x, and y. It performs the following steps when called:

  1. Assert: op is &, ^, or |.
  2. Let lnum be ! ToInt32(x).
  3. Let rnum be ! ToInt32(y).
  4. Let lbits be the 32-bit two's complement bit string representing(lnum).
  5. Let rbits be the 32-bit two's complement bit string representing(rnum).
  6. If op is &, let result be the result of applying the bitwise AND operation to lbits and rbits.
  7. Else if op is ^, let result be the result of applying the bitwise exclusive OR (XOR) operation to lbits and rbits.
  8. Else, op is |. Let result be the result of applying the bitwise inclusive OR operation to lbits and rbits.
  9. Return theNumber valuefor theintegerrepresented by the 32-bit two's complement bit string result.

6.1.6.1.17 Number::bitwiseAND ( x, y )

The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. ReturnNumberBitwiseOp(&, x, y).

6.1.6.1.18 Number::bitwiseXOR ( x, y )

The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. ReturnNumberBitwiseOp(^, x, y).

6.1.6.1.19 Number::bitwiseOR ( x, y )

The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a Number). It performs the following steps when called:

  1. ReturnNumberBitwiseOp(|, x, y).

6.1.6.1.20 Number::toString ( x )

The abstract operation Number::toString takes argument x (a Number). It converts x to String format. It performs the following steps when called:

  1. If x isNaN, return the String"NaN".
  2. If x is+0𝔽 or-0𝔽, return the String"0".
  3. If x <+0𝔽, return thestring-concatenationof"-"and !Number::toString(-x).
  4. If x is+∞𝔽, return the String"Infinity".
  5. Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1s < 10k,𝔽(s × 10n - k) is x, and k is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is not divisible by 10, and that the least significant digit of s is not necessarily uniquely determined by these criteria.
  6. If kn ≤ 21, return thestring-concatenationof:
    • the code units of the k digits of the decimal representation of s (in order, with no leading zeroes)
    • n - k occurrences of the code unit 0x0030 (DIGIT ZERO)
  7. If 0 < n ≤ 21, return thestring-concatenationof:
    • the code units of the most significant n digits of the decimal representation of s
    • the code unit 0x002E (FULL STOP)
    • the code units of the remaining k - n digits of the decimal representation of s
  8. If -6 < n ≤ 0, return thestring-concatenationof:
    • the code unit 0x0030 (DIGIT ZERO)
    • the code unit 0x002E (FULL STOP)
    • -n occurrences of the code unit 0x0030 (DIGIT ZERO)
    • the code units of the k digits of the decimal representation of s
  9. Otherwise, if k = 1, return thestring-concatenationof:
    • the code unit of the single digit of s
    • the code unit 0x0065 (LATIN SMALL LETTER E)
    • the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n - 1 is positive or negative
    • the code units of the decimal representation of theintegerabs(n - 1) (with no leading zeroes)
  10. Return thestring-concatenationof:
    • the code units of the most significant digit of the decimal representation of s
    • the code unit 0x002E (FULL STOP)
    • the code units of the remaining k - 1 digits of the decimal representation of s
    • the code unit 0x0065 (LATIN SMALL LETTER E)
    • the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS) according to whether n - 1 is positive or negative
    • the code units of the decimal representation of theintegerabs(n - 1) (with no leading zeroes)
Note 1

The following observations may be useful as guidelines for implementations, but are not part of the normative requirements of this Standard:

  • If x is anyNumber valueother than-0𝔽, thenToNumber(ToString(x)) is exactly the sameNumber valueas x.
  • The least significant digit of s is not always uniquely determined by the requirements listed in step5.
Note 2

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step5be used as a guideline:

  1. Otherwise, let n, k, and s be integers such that k ≥ 1, 10k - 1s < 10k,𝔽(s × 10n - k) is x, and k is as small as possible. If there are multiple possibilities for s, choose the value of s for which s × 10n - k is closest in value to(x). If there are two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation of s and that s is not divisible by 10.
Note 3

Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). 30 November 1990. Available as
http://ampl.com/REFS/abstracts.html#rounding. Associated code available as
http://netlib.sandia.gov/fp/dtoa.c and as
http://netlib.sandia.gov/fp/g_fmt.c and may also be found at the various netlib mirror sites.

6.1.6.2 The BigInt Type

The BigInt type represents anintegervalue. The value may be any size and is not limited to a particular bit-width. Generally, where not otherwise noted, operations are designed to return exact mathematically-based answers. For binary operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits set infinitely to the left.

The BigInt::unit value is1.

6.1.6.2.1 BigInt::unaryMinus ( x )

The abstract operation BigInt::unaryMinus takes argument x (a BigInt). It performs the following steps when called:

  1. If x is0, return0.
  2. Return the BigInt value that represents the negation of(x).

6.1.6.2.2 BigInt::bitwiseNOT ( x )

The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt). It returns the one's complement of x; that is, -x -1.

6.1.6.2.3 BigInt::exponentiate ( base, exponent )

The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and exponent (a BigInt). It performs the following steps when called:

  1. If exponent <0, throw aRangeErrorexception.
  2. If base is0 and exponent is0, return1.
  3. Return the BigInt value that represents(base) raised to the power(exponent).

6.1.6.2.4 BigInt::multiply ( x, y )

The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the result of multiplying x and y.

Note
Even if the result has a much larger bit width than the input, the exact mathematical answer is given.

6.1.6.2.5 BigInt::divide ( x, y )

The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. If y is0, throw aRangeErrorexception.
  2. Let quotient be(x) /(y).
  3. Return the BigInt value that represents quotient rounded towards 0 to the nextintegervalue.

6.1.6.2.6 BigInt::remainder ( n, d )

The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a BigInt). It performs the following steps when called:

  1. If d is0, throw aRangeErrorexception.
  2. If n is0, return0.
  3. Let r be the BigInt defined by the mathematical relation r = n - (d × q) where q is a BigInt that is negative only if n/d is negative and positive only if n/d is positive, and whose magnitude is as large as possible without exceeding the magnitude of the true mathematical quotient of n and d.
  4. Return r.
Note
The sign of the result equals the sign of the dividend.

6.1.6.2.7 BigInt::add ( x, y )

The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the sum of x and y.

6.1.6.2.8 BigInt::subtract ( x, y )

The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a BigInt). It returns the BigInt value that represents the difference x minus y.

6.1.6.2.9 BigInt::leftShift ( x, y )

The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. If y <0, then
    1. Return the BigInt value that represents(x) / 2-y, rounding down to the nearestinteger, including for negative numbers.
  2. Return the BigInt value that represents(x) × 2y.
Note
Semantics here should be equivalent to a bitwise shift, treating the BigInt as an infinite length string of binary two's complement digits.

6.1.6.2.10 BigInt::signedRightShift ( x, y )

The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigInt::leftShift(x, -y).

6.1.6.2.11 BigInt::unsignedRightShift ( x, y )

The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. Throw aTypeErrorexception.

6.1.6.2.12 BigInt::lessThan ( x, y )

The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a BigInt). It returnstrueif(x) <(y) andfalseotherwise.

6.1.6.2.13 BigInt::equal ( x, y )

The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a BigInt). It returnstrueif(x) =(y) andfalseotherwise.

6.1.6.2.14 BigInt::sameValue ( x, y )

The abstract operation BigInt::sameValue takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigInt::equal(x, y).

6.1.6.2.15 BigInt::sameValueZero ( x, y )

The abstract operation BigInt::sameValueZero takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigInt::equal(x, y).

6.1.6.2.16 BinaryAnd ( x, y )

The abstract operation BinaryAnd takes arguments x and y. It performs the following steps when called:

  1. Assert: x is 0 or 1.
  2. Assert: y is 0 or 1.
  3. If x is 1 and y is 1, return 1.
  4. Else, return 0.

6.1.6.2.17 BinaryOr ( x, y )

The abstract operation BinaryOr takes arguments x and y. It performs the following steps when called:

  1. Assert: x is 0 or 1.
  2. Assert: y is 0 or 1.
  3. If x is 1 or y is 1, return 1.
  4. Else, return 0.

6.1.6.2.18 BinaryXor ( x, y )

The abstract operation BinaryXor takes arguments x and y. It performs the following steps when called:

  1. Assert: x is 0 or 1.
  2. Assert: y is 0 or 1.
  3. If x is 1 and y is 0, return 1.
  4. Else if x is 0 and y is 1, return 1.
  5. Else, return 0.

6.1.6.2.19 BigIntBitwiseOp ( op, x, y )

The abstract operation BigIntBitwiseOp takes arguments op (a sequence of Unicode code points), x (a BigInt), and y (a BigInt). It performs the following steps when called:

  1. Assert: op is &, ^, or |.
  2. Set x to(x).
  3. Set y to(y).
  4. Let result be 0.
  5. Let shift be 0.
  6. Repeat, until (x = 0 or x = -1) and (y = 0 or y = -1),
    1. Let xDigit be xmodulo2.
    2. Let yDigit be ymodulo2.
    3. If op is &, set result to result + 2shift ×BinaryAnd(xDigit, yDigit).
    4. Else if op is |, set result to result + 2shift ×BinaryOr(xDigit, yDigit).
    5. Else,
      1. Assert: op is ^.
      2. Set result to result + 2shift ×BinaryXor(xDigit, yDigit).
    6. Set shift to shift + 1.
    7. Set x to (x - xDigit) / 2.
    8. Set y to (y - yDigit) / 2.
  7. If op is &, let tmp beBinaryAnd(xmodulo2, ymodulo2).
  8. Else if op is |, let tmp beBinaryOr(xmodulo2, ymodulo2).
  9. Else,
    1. Assert: op is ^.
    2. Let tmp beBinaryXor(xmodulo2, ymodulo2).
  10. If tmp ≠ 0, then
    1. Set result to result - 2shift.
    2. NOTE: This extends the sign.
  11. Return the BigInt value for result.

6.1.6.2.20 BigInt::bitwiseAND ( x, y )

The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigIntBitwiseOp(&, x, y).

6.1.6.2.21 BigInt::bitwiseXOR ( x, y )

The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigIntBitwiseOp(^, x, y).

6.1.6.2.22 BigInt::bitwiseOR ( x, y )

The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a BigInt). It performs the following steps when called:

  1. ReturnBigIntBitwiseOp(|, x, y).

6.1.6.2.23 BigInt::toString ( x )

The abstract operation BigInt::toString takes argument x (a BigInt). It converts x to String format. It performs the following steps when called:

  1. If x <0, return thestring-concatenationof the String"-"and !BigInt::toString(-x).
  2. Return the String value consisting of the code units of the digits of the decimal representation of x.

6.1.7 The Object Type

An Object is logically a collection of properties. Each property is either a data property, or an accessor property:

  • A data property associates a key value with anECMAScript language valueand a set of Boolean attributes.
  • An accessor property associates a key value with one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store or retrieve anECMAScript language valuethat is associated with the property.

Properties are identified using key values. A property key value is either an ECMAScript String value or a Symbol value. All String and Symbol values, including the empty String, are valid as property keys. A property name is a property key that is a String value.

An integer index is a String-valued property key that is a canonical numeric String (see7.1.21) and whose numeric value is either+0𝔽 or a positiveintegral Number𝔽(253 - 1). An array index is aninteger indexwhose numeric value i is in the range+0𝔽i <𝔽(232 - 1).

Property keys are used to access properties and their values. There are two kinds of access for properties: get and set, corresponding to value retrieval and assignment, respectively. The properties accessible via get and set access includes both own properties that are a direct part of an object and inherited properties which are provided by another associated object via a property inheritance relationship. Inherited properties may be either own or inherited properties of the associated object. Each own property of an object must each have a key value that is distinct from the key values of the other own properties of that object.

All objects are logically collections of properties, but there are multiple forms of objects that differ in their semantics for accessing and manipulating their properties. Please see6.1.7.2for definitions of the multiple forms of objects.

6.1.7.1 Property Attributes

Attributes are used in this specification to define and explain the state of Object properties. Adata propertyassociates a key value with the attributes listed inTable 3.

Table 3: Attributes of a Data Property
Attribute NameValue DomainDescription
[[Value]]AnyECMAScript language typeThe value retrieved by a get access of the property.
[[Writable]]BooleanIffalse, attempts by ECMAScript code to change the property's [[Value]] attribute using [[Set]] will not succeed.
[[Enumerable]]BooleanIftrue, the property will be enumerated by a for-in enumeration (see14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]BooleanIffalse, attempts to delete the property, change the property to be anaccessor property, or change its attributes (other than [[Value]], or changing [[Writable]] tofalse) will fail.

Anaccessor propertyassociates a key value with the attributes listed inTable 4.

Table 4: Attributes of an Accessor Property
Attribute NameValue DomainDescription
[[Get]]Object | UndefinedIf the value is an Object it must be afunction object. The function's [[Call]] internal method (Table 7) is called with an empty arguments list to retrieve the property value each time a get access of the property is performed.
[[Set]]Object | UndefinedIf the value is an Object it must be afunction object. The function's [[Call]] internal method (Table 7) is called with an arguments list containing the assigned value as its sole argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the value returned by subsequent calls to the property's [[Get]] internal method.
[[Enumerable]]BooleanIftrue, the property is to be enumerated by a for-in enumeration (see14.7.5). Otherwise, the property is said to be non-enumerable.
[[Configurable]]BooleanIffalse, attempts to delete the property, change the property to be adata property, or change its attributes will fail.

If the initial values of a property's attributes are not explicitly specified by this specification, the default value defined inTable 5is used.

Table 5: Default Attribute Values
Attribute NameDefault Value
[[Value]]undefined
[[Get]]undefined
[[Set]]undefined
[[Writable]]false
[[Enumerable]]false
[[Configurable]]false

6.1.7.2 Object Internal Methods and Internal Slots

The actual semantics of objects, in ECMAScript, are specified via algorithms called internal methods. Each object in an ECMAScript engine is associated with a set of internal methods that defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are defined by this specification purely for expository purposes. However, each object within an implementation of ECMAScript must behave as specified by the internal methods associated with it. The exact manner in which this is accomplished is determined by the implementation.

Internal method names are polymorphic. This means that different object values may perform different algorithms when a common internal method name is invoked upon them. That actual object upon which an internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an algorithm attempts to use an internal method of an object that the object does not support, aTypeErrorexception is thrown.

Internal slots correspond to internal state that is associated with objects and used by various ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited. Depending upon the specific internal slot specification, such state may consist of values of anyECMAScript language typeor of specific ECMAScript specification type values. Unless explicitly specified otherwise, internal slots are allocated as part of the process of creating an object and may not be dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the valueundefined. Various algorithms within this specification create objects that have internal slots. However, the ECMAScript language provides no direct way to associate internal slots with an object.

All objects have an internal slot named [[PrivateElements]], which is aListof PrivateElements. ThisListrepresents the values of the private fields, methods, and accessors for the object. Initially, it is an emptyList.

Internal methods and internal slots are identified within this specification using names enclosed in double square brackets [[ ]].

Table 6summarizes the essential internal methods used by this specification that are applicable to all objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the essential internal methods. However, all objects do not necessarily use the same algorithms for those methods.

An ordinary object is an object that satisfies all of the following criteria:

  • For the internal methods listed inTable 6, the object uses those defined in10.1.
  • If the object has a [[Call]] internal method, it uses the one defined in10.2.1.
  • If the object has a [[Construct]] internal method, it uses the one defined in10.2.2.

An exotic object is an object that is not anordinary object.

This specification recognizes different kinds of exotic objects by those objects' internal methods. An object that is behaviourally equivalent to a particular kind ofexotic object(such as anArray exotic objector abound function exotic object), but does not have the same collection of internal methods specified for that kind, is not recognized as that kind ofexotic object.

The “Signature” column ofTable 6and other similar tables describes the invocation pattern for each internal method. The invocation pattern always includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an ECMAScript type name then the name describes the required type of the parameter value. If an internal method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of the returned value. The type names used in signatures refer to the types defined in clause6augmented by the following additional names. “any” means the value may be anyECMAScript language type.

In addition to its parameters, an internal method always has access to the object that is the target of the method invocation.

An internal method implicitly returns aCompletion Record, either anormal completionthat wraps a value of the return type shown in its invocation pattern, or athrow completion.

Table 6: Essential Internal Methods
Internal MethodSignatureDescription
[[GetPrototypeOf]]( ) Object | NullDetermine the object that provides inherited properties for this object. Anullvalue indicates that there are no inherited properties.
[[SetPrototypeOf]](Object | Null) BooleanAssociate this object with another object that provides inherited properties. Passingnullindicates that there are no inherited properties. Returnstrueindicating that the operation was completed successfully orfalseindicating that the operation was not successful.
[[IsExtensible]]( ) BooleanDetermine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]( ) BooleanControl whether new properties may be added to this object. Returnstrueif the operation was successful orfalseif the operation was unsuccessful.
[[GetOwnProperty]](propertyKey) Undefined |Property DescriptorReturn aProperty Descriptorfor the own property of this object whose key is propertyKey, orundefinedif no such property exists.
[[DefineOwnProperty]](propertyKey, PropertyDescriptor) BooleanCreate or alter the own property, whose key is propertyKey, to have the state described by PropertyDescriptor. Returntrueif that property was successfully created/updated orfalseif the property could not be created or updated.
[[HasProperty]](propertyKey) BooleanReturn a Boolean value indicating whether this object already has either an own or inherited property whose key is propertyKey.
[[Get]](propertyKey, Receiver) anyReturn the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the property value, Receiver is used as thethisvalue when evaluating the code.
[[Set]](propertyKey, value, Receiver) BooleanSet the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value, Receiver is used as thethisvalue when evaluating the code. Returnstrueif the property value was set orfalseif it could not be set.
[[Delete]](propertyKey) BooleanRemove the own property whose key is propertyKey from this object. Returnfalseif the property was not deleted and is still present. Returntrueif the property was deleted or is not present.
[[OwnPropertyKeys]]( ) Listof propertyKeyReturn aListwhose elements are all of the own property keys for the object.

Table 7summarizes additional essential internal methods that are supported by objects that may be called as functions. A function object is an object that supports the [[Call]] internal method. A constructor is an object that supports the [[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, everyconstructormust be afunction object. Therefore, aconstructormay also be referred to as a constructorfunction or constructorfunction object.

Table 7: Additional Essential Internal Methods of Function Objects
Internal MethodSignatureDescription
[[Call]](any, aListof any) anyExecutes code associated with this object. Invoked via a function call expression. The arguments to the internal method are athisvalue and aListwhose elements are the arguments passed to the function by a call expression. Objects that implement this internal method are callable.
[[Construct]](aListof any, Object) ObjectCreates an object. Invoked via the new operator or a super call. The first argument to the internal method is aListwhose elements are the arguments of theconstructorinvocation or the super call. The second argument is the object to which the new operator was initially applied. Objects that implement this internal method are called constructors. Afunction objectis not necessarily aconstructorand such non-constructorfunction objects do not have a [[Construct]] internal method.

The semantics of the essential internal methods for ordinary objects and standard exotic objects are specified in clause10. If any specified use of an internal method of anexotic objectis not supported by an implementation, that usage must throw aTypeErrorexception when attempted.

6.1.7.3 Invariants of the Essential Internal Methods

The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic objects in this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.

Any implementation provided exotic objects must also maintain these invariants for those objects. Violation of these invariants may cause ECMAScript code to have unpredictable behaviour and create security issues. However, violation of these invariants must never compromise the memory safety of an implementation.

An implementation must not allow these invariants to be circumvented in any manner such as by providing alternative interfaces that implement the functionality of the essential internal methods without enforcing their invariants.

Definitions:

  • The target of an internal method is the object upon which the internal method is called.
  • A target is non-extensible if it has been observed to returnfalsefrom its [[IsExtensible]] internal method, ortruefrom its [[PreventExtensions]] internal method.
  • A non-existent property is a property that does not exist as an own property on a non-extensible target.
  • All references to SameValue are according to the definition of theSameValuealgorithm.

Return value:

The value returned by any internal method must be aCompletion Recordwith either:

  • [[Type]] =normal, [[Target]] =empty, and [[Value]] = a value of the "normal return type" shown below for that internal method, or
  • [[Type]] =throw, [[Target]] =empty, and [[Value]] = anyECMAScript language value.
Note 1

An internal method must not return a completion with [[Type]] =continue,break, orreturn.

[[GetPrototypeOf]] ( )

  • The normal return type is either Object or Null.
  • If target is non-extensible, and [[GetPrototypeOf]] returns a value V, then any future calls to [[GetPrototypeOf]] should return theSameValueas V.
Note 2

An object's prototype chain should have finite length (that is, starting from any object, recursively applying the [[GetPrototypeOf]] internal method to its result should eventually lead to the valuenull). However, this requirement is not enforceable as an object level invariant if the prototype chain includes any exotic objects that do not use theordinary objectdefinition of [[GetPrototypeOf]]. Such a circular prototype chain may result in infinite loops when accessing object properties.

[[SetPrototypeOf]] ( V )

  • The normal return type is Boolean.
  • If target is non-extensible, [[SetPrototypeOf]] must returnfalse, unless V is theSameValueas the target's observed [[GetPrototypeOf]] value.

[[IsExtensible]] ( )

  • The normal return type is Boolean.
  • If [[IsExtensible]] returnsfalse, all future calls to [[IsExtensible]] on the target must returnfalse.

[[PreventExtensions]] ( )

  • The normal return type is Boolean.
  • If [[PreventExtensions]] returnstrue, all future calls to [[IsExtensible]] on the target must returnfalseand the target is now considered non-extensible.

[[GetOwnProperty]] ( P )

  • The normal return type is eitherProperty Descriptoror Undefined.
  • If the Type of the return value isProperty Descriptor, the return value must be a fully populatedProperty Descriptor.
  • If P is described as a non-configurable, non-writable owndata property, all future calls to [[GetOwnProperty]] ( P ) must returnProperty Descriptorwhose [[Value]] isSameValueas P's [[Value]] attribute.
  • If P's attributes other than [[Writable]] may change over time or if the property might be deleted, then P's [[Configurable]] attribute must betrue.
  • If the [[Writable]] attribute may change fromfalsetotrue, then the [[Configurable]] attribute must betrue.
  • If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as non-existent (i.e. [[GetOwnProperty]] (P) must returnundefined).
Note 3

As a consequence of the third invariant, if a property is described as adata propertyand it may return different values over time, then either or both of the [[Writable]] and [[Configurable]] attributes must betrueeven if no mechanism to change the value is exposed via the other essential internal methods.

[[DefineOwnProperty]] ( P, Desc )

  • The normal return type is Boolean.
  • [[DefineOwnProperty]] must returnfalseif P has previously been observed as a non-configurable own property of the target, unless either:
    1. P is a writabledata property. A non-configurable writabledata propertycan be changed into a non-configurable non-writabledata property.
    2. All attributes of Desc are theSameValueas P's attributes.
  • [[DefineOwnProperty]] (P, Desc) must returnfalseif target is non-extensible and P is a non-existent own property. That is, a non-extensible target object cannot be extended with new properties.

[[HasProperty]] ( P )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable own data oraccessor propertyof the target, [[HasProperty]] must returntrue.

[[Get]] ( P, Receiver )

  • The normal return type is anyECMAScript language type.
  • If P was previously observed as a non-configurable, non-writable owndata propertyof the target with value V, then [[Get]] must return theSameValueas V.
  • If P was previously observed as a non-configurable ownaccessor propertyof the target whose [[Get]] attribute isundefined, the [[Get]] operation must returnundefined.

[[Set]] ( P, V, Receiver )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable, non-writable owndata propertyof the target, then [[Set]] must returnfalseunless V is theSameValueas P's [[Value]] attribute.
  • If P was previously observed as a non-configurable ownaccessor propertyof the target whose [[Set]] attribute isundefined, the [[Set]] operation must returnfalse.

[[Delete]] ( P )

  • The normal return type is Boolean.
  • If P was previously observed as a non-configurable own data oraccessor propertyof the target, [[Delete]] must returnfalse.

[[OwnPropertyKeys]] ( )

  • The normal return type isList.
  • The returnedListmust not contain any duplicate entries.
  • The Type of each element of the returnedListis either String or Symbol.
  • The returnedListmust contain at least the keys of all non-configurable own properties that have previously been observed.
  • If the target is non-extensible, the returnedListmust contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].

[[Call]] ( )

[[Construct]] ( )

  • The normal return type is Object.
  • The target must also have a [[Call]] internal method.

6.1.7.4 Well-Known Intrinsic Objects

Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this specification and which usually haverealm-specific identities. Unless otherwise specified each intrinsic object actually corresponds to a set of similar objects, one perrealm.

Within this specification a reference such as %name% means the intrinsic object, associated with the currentrealm, corresponding to the name. A reference such as %name.a.b% means, as if the "b" property of the "a" property of the intrinsic object %name% was accessed prior to any ECMAScript code being evaluated. Determination of the currentrealmand its intrinsics is described in9.4. The well-known intrinsics are listed inTable 8.

Table 8: Well-Known Intrinsic Objects
Intrinsic NameGlobal NameECMAScript Language Association
%AggregateError%AggregateErrorThe AggregateErrorconstructor(20.5.7.1)
%Array%ArrayThe Arrayconstructor(23.1.1)
%ArrayBuffer%ArrayBufferThe ArrayBufferconstructor(25.1.3)
%ArrayIteratorPrototype%The prototype of Array iterator objects (23.1.5)
%AsyncFromSyncIteratorPrototype%The prototype of async-from-sync iterator objects (27.1.4)
%AsyncFunction%Theconstructorof async function objects (27.7.1)
%AsyncGeneratorFunction%Theconstructorof async iterator objects (27.4.1)
%AsyncIteratorPrototype%An object that all standard built-in async iterator objects indirectly inherit from
%Atomics%AtomicsThe Atomics object (25.4)
%BigInt%BigIntThe BigIntconstructor(21.2.1)
%BigInt64Array%BigInt64ArrayThe BigInt64Arrayconstructor(23.2)
%BigUint64Array%BigUint64ArrayThe BigUint64Arrayconstructor(23.2)
%Boolean%BooleanThe Booleanconstructor(20.3.1)
%DataView%DataViewThe DataViewconstructor(25.3.2)
%Date%DateThe Dateconstructor(21.4.2)
%decodeURI%decodeURIThe decodeURI function (19.2.6.2)
%decodeURIComponent%decodeURIComponentThe decodeURIComponent function (19.2.6.3)
%encodeURI%encodeURIThe encodeURI function (19.2.6.4)
%encodeURIComponent%encodeURIComponentThe encodeURIComponent function (19.2.6.5)
%Error%ErrorThe Errorconstructor(20.5.1)
%eval%evalThe eval function (19.2.1)
%EvalError%EvalErrorThe EvalErrorconstructor(20.5.5.1)
%FinalizationRegistry%FinalizationRegistryTheFinalizationRegistryconstructor(26.2.1)
%Float32Array%Float32ArrayThe Float32Arrayconstructor(23.2)
%Float64Array%Float64ArrayThe Float64Arrayconstructor(23.2)
%ForInIteratorPrototype%The prototype of For-In iterator objects (14.7.5.10)
%Function%FunctionThe Functionconstructor(20.2.1)
%GeneratorFunction%Theconstructorof generator objects (27.3.1)
%Int8Array%Int8ArrayThe Int8Arrayconstructor(23.2)
%Int16Array%Int16ArrayThe Int16Arrayconstructor(23.2)
%Int32Array%Int32ArrayThe Int32Arrayconstructor(23.2)
%isFinite%isFiniteThe isFinite function (19.2.2)
%isNaN%isNaNThe isNaN function (19.2.3)
%IteratorPrototype%An object that all standard built-in iterator objects indirectly inherit from
%JSON%JSONThe JSON object (25.5)
%Map%MapThe Mapconstructor(24.1.1)
%MapIteratorPrototype%The prototype of Map iterator objects (24.1.5)
%Math%MathThe Math object (21.3)
%Number%NumberThe Numberconstructor(21.1.1)
%Object%ObjectThe Objectconstructor(20.1.1)
%parseFloat%parseFloatThe parseFloat function (19.2.4)
%parseInt%parseIntThe parseInt function (19.2.5)
%Promise%PromiseThe Promiseconstructor(27.2.3)
%Proxy%ProxyThe Proxyconstructor(28.2.1)
%RangeError%RangeErrorThe RangeErrorconstructor(20.5.5.2)
%ReferenceError%ReferenceErrorThe ReferenceErrorconstructor(20.5.5.3)
%Reflect%ReflectThe Reflect object (28.1)
%RegExp%RegExpThe RegExpconstructor(22.2.3)
%RegExpStringIteratorPrototype%The prototype of RegExp String Iterator objects (22.2.7)
%Set%SetThe Setconstructor(24.2.1)
%SetIteratorPrototype%The prototype of Set iterator objects (24.2.5)
%SharedArrayBuffer%SharedArrayBufferThe SharedArrayBufferconstructor(25.2.2)
%String%StringThe Stringconstructor(22.1.1)
%StringIteratorPrototype%The prototype of String iterator objects (22.1.5)
%Symbol%SymbolThe Symbolconstructor(20.4.1)
%SyntaxError%SyntaxErrorThe SyntaxErrorconstructor(20.5.5.4)
%ThrowTypeError%Afunction objectthat unconditionally throws a new instance of%TypeError%
%TypedArray%The super class of all typed Array constructors (23.2.1)
%TypeError%TypeErrorThe TypeErrorconstructor(20.5.5.5)
%Uint8Array%Uint8ArrayThe Uint8Arrayconstructor(23.2)
%Uint8ClampedArray%Uint8ClampedArrayThe Uint8ClampedArrayconstructor(23.2)
%Uint16Array%Uint16ArrayThe Uint16Arrayconstructor(23.2)
%Uint32Array%Uint32ArrayThe Uint32Arrayconstructor(23.2)
%URIError%URIErrorThe URIErrorconstructor(20.5.5.6)
%WeakMap%WeakMapThe WeakMapconstructor(24.3.1)
%WeakRef%WeakRefTheWeakRefconstructor(26.1.1)
%WeakSet%WeakSetThe WeakSetconstructor(24.4.1)
Note

Additional entries inTable 85.

6.2 ECMAScript Specification Types

A specification type corresponds to meta-values that are used within algorithms to describe the semantics of ECMAScript language constructs and ECMAScript language types. The specification types include Reference,List,Completion,Property Descriptor,Environment Record,Abstract Closure, andData Block. Specification type values are specification artefacts that do not necessarily correspond to any specific entity within an ECMAScript implementation. Specification type values may be used to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of ECMAScript language variables.

6.2.1 The List and Record Specification Types

The List type is used to explain the evaluation of argument lists (see13.3.8) in new expressions, in function calls, and in other algorithms where a simple ordered list of values is needed. Values of the List type are simply ordered sequences of list elements containing the individual values. These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin indices. For notational convenience an array-like syntax can be used to access List elements. For example, arguments[2] is shorthand for saying the 3rd element of the List arguments.

When an algorithm iterates over the elements of a List without specifying an order, the order used is the order of the elements in the List.

For notational convenience within this specification, a literal syntax can be used to express a new List value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a specific value. A new empty List can be expressed as « ».

In this specification, the phrase "the list-concatenation of A, B, ..." (where each argument is a possibly empty List) denotes a new List value whose elements are the concatenation of the elements (in order) of each of the arguments (in order).

The Record type is used to describe data aggregations within the algorithms of this specification. A Record type value consists of one or more named fields. The value of each field is either an ECMAScript value or an abstract value represented by a name associated with the Record type. Field names are always enclosed in double brackets, for example [[Value]].

For notational convenience within this specification, an object literal-like syntax can be used to express a Record value. For example, { [[Field1]]: 42, [[Field2]]:false, [[Field3]]:empty} defines a Record value that has three fields, each of which is initialized to a specific value. Field name order is not significant. Any fields that are not explicitly listed are considered to be absent.

In specification text and algorithms, dot notation may be used to refer to a specific field of a Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.

Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a literal Record value to identify the specific kind of aggregations that is being described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]:false, [[Configurable]]:true}.

6.2.2 The Set and Relation Specification Types

The Set type is used to explain a collection of unordered elements for use in thememory model. Values of the Set type are simple collections of elements, where no element appears more than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted from each other.

The Relation type is used to explain constraints on Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a Relation on events is a set of ordered pairs of events. For a Relation R and two values a and b in the value domain of R, a R b is shorthand for saying the ordered pair (a, b) is a member of R. A Relation is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.

A strict partial order is a Relation value R that satisfies the following.

  • For all a, b, and c in R's domain:

    • It is not the case that a R a, and
    • If a R b and b R c, then a R c.
Note 1

The two properties above are called irreflexivity and transitivity, respectively.

A strict total order is a Relation value R that satisfies the following.

  • For all a, b, and c in R's domain:

    • a is identical to b or a R b or b R a, and
    • It is not the case that a R a, and
    • If a R b and b R c, then a R c.
Note 2

The three properties above are called totality, irreflexivity, and transitivity, respectively.

6.2.3 The Completion Record Specification Type

The Completion type is aRecordused to explain the runtime propagation of values and control flow such as the behaviour of statements (break, continue, return and throw) that perform nonlocal transfers of control.

Values of the Completion type areRecordvalues whose fields are defined byTable 9. Such values are referred to as Completion Records.

Table 9:Completion RecordFields
Field NameValueMeaning
[[Type]]One ofnormal,break,continue,return, orthrowThe type of completion that occurred.
[[Value]]anyECMAScript language valueoremptyThe value that was produced.
[[Target]]any ECMAScript string oremptyThe target label for directed control transfers.

The following shorthand terms are sometimes used to refer to completions.

  • normal completion refers to any completion with a [[Type]] value ofnormal.
  • break completion refers to any completion with a [[Type]] value ofbreak.
  • continue completion refers to any completion with a [[Type]] value ofcontinue.
  • return completion refers to any completion with a [[Type]] value ofreturn.
  • throw completion refers to any completion with a [[Type]] value ofthrow.
  • abrupt completion refers to any completion with a [[Type]] value other thannormal.

Callable objects that are defined in this specification only return a normal completion or a throw completion. Returning any other kind of completion is considered an editorial error.

Implementation-definedcallable objects must return either a normal completion or a throw completion.

6.2.3.1 Await

Algorithm steps that say

  1. Let completion beAwait(value).

mean the same thing as:

  1. Let asyncContext be therunning execution context.
  2. Let promise be ? PromiseResolve(%Promise%, value).
  3. Let fulfilledClosure be a newAbstract Closurewith parameters (value) that captures asyncContext and performs the following steps when called:
    1. Let prevContext be therunning execution context.
    2. Suspend prevContext.
    3. Push asyncContext onto theexecution context stack; asyncContext is now therunning execution context.
    4. Resume the suspended evaluation of asyncContext usingNormalCompletion(value) as the result of the operation that suspended it.
    5. Assert: When we reach this step, asyncContext has already been removed from theexecution context stackand prevContext is the currentlyrunning execution context.
    6. Returnundefined.
  4. Let onFulfilled be ! CreateBuiltinFunction(fulfilledClosure, 1,"", « »).
  5. Let rejectedClosure be a newAbstract Closurewith parameters (reason) that captures asyncContext and performs the following steps when called:
    1. Let prevContext be therunning execution context.
    2. Suspend prevContext.
    3. Push asyncContext onto theexecution context stack; asyncContext is now therunning execution context.
    4. Resume the suspended evaluation of asyncContext usingThrowCompletion(reason) as the result of the operation that suspended it.
    5. Assert: When we reach this step, asyncContext has already been removed from theexecution context stackand prevContext is the currentlyrunning execution context.
    6. Returnundefined.
  6. Let onRejected be ! CreateBuiltinFunction(rejectedClosure, 1,"", « »).
  7. Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).
  8. Remove asyncContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
  9. Set the code evaluation state of asyncContext such that when evaluation is resumed with aCompletioncompletion, the following steps of the algorithm that invokedAwaitwill be performed, with completion available.
  10. Return.
  11. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of asyncContext.

where all aliases in the above steps, with the exception of completion, are ephemeral and visible only in the steps pertaining to Await.

Note

Await can be combined with the ? and ! prefixes, so that for example

  1. Let result be ? Await(value).

means the same thing as:

  1. Let result beAwait(value).
  2. ReturnIfAbrupt(result).

6.2.3.2 NormalCompletion

The abstract operation NormalCompletion with a single argument, such as:

  1. ReturnNormalCompletion(argument).

Is a shorthand that is defined as follows:

  1. ReturnCompletion{ [[Type]]:normal, [[Value]]: argument, [[Target]]:empty}.

6.2.3.3 ThrowCompletion

The abstract operation ThrowCompletion with a single argument, such as:

  1. ReturnThrowCompletion(argument).

Is a shorthand that is defined as follows:

  1. ReturnCompletion{ [[Type]]:throw, [[Value]]: argument, [[Target]]:empty}.

6.2.3.4 UpdateEmpty ( completionRecord, value )

The abstract operation UpdateEmpty takes arguments completionRecord and value. It performs the following steps when called:

  1. Assert: If completionRecord.[[Type]] is eitherreturnorthrow, then completionRecord.[[Value]] is notempty.
  2. If completionRecord.[[Value]] is notempty, returnCompletion(completionRecord).
  3. ReturnCompletion{ [[Type]]: completionRecord.[[Type]], [[Value]]: value, [[Target]]: completionRecord.[[Target]] }.

6.2.4 The Reference Record Specification Type

The Reference Record type is used to explain the behaviour of such operators as delete, typeof, the assignment operators, the superkeywordand other language features. For example, the left-hand operand of an assignment is expected to produce a Reference Record.

A Reference Record is a resolved name or property binding; its fields are defined byTable 10.

Table 10:Reference RecordFields
Field NameValueMeaning
[[Base]]One of:The value orEnvironment Recordwhich holds the binding. A [[Base]] ofunresolvableindicates that the binding could not be resolved.
[[ReferencedName]]String, Symbol, orPrivate NameThe name of the binding. Always a String if [[Base]] value is anEnvironment Record.
[[Strict]]Booleantrueif theReference Recordoriginated instrict mode code,falseotherwise.
[[ThisValue]]anyECMAScript language valueoremptyIf notempty, theReference Recordrepresents a property binding that was expressed using the superkeyword; it is called a Super Reference Record and its [[Base]] value will never be anEnvironment Record. In that case, the [[ThisValue]] field holds thethisvalue at the time theReference Recordwas created.

The followingabstract operationsare used in this specification to operate upon Reference Records:

6.2.4.1 IsPropertyReference ( V )

The abstract operation IsPropertyReference takes argument V. It performs the following steps when called:

  1. Assert: V is aReference Record.
  2. If V.[[Base]] isunresolvable, returnfalse.
  3. IfType(V.[[Base]]) is Boolean, String, Symbol, BigInt, Number, or Object, returntrue; otherwise returnfalse.

6.2.4.2 IsUnresolvableReference ( V )

The abstract operation IsUnresolvableReference takes argument V. It performs the following steps when called:

  1. Assert: V is aReference Record.
  2. If V.[[Base]] isunresolvable, returntrue; otherwise returnfalse.

6.2.4.3 IsSuperReference ( V )

The abstract operation IsSuperReference takes argument V. It performs the following steps when called:

  1. Assert: V is aReference Record.
  2. If V.[[ThisValue]] is notempty, returntrue; otherwise returnfalse.

6.2.4.4 IsPrivateReference ( V )

The abstract operation IsPrivateReference takes argument V. It performs the following steps when called:

  1. Assert: V is aReference Record.
  2. If V.[[ReferencedName]] is aPrivate Name, returntrue; otherwise returnfalse.

6.2.4.5 GetValue ( V )

The abstract operation GetValue takes argument V. It performs the following steps when called:

  1. ReturnIfAbrupt(V).
  2. If V is not aReference Record, return V.
  3. IfIsUnresolvableReference(V) istrue, throw aReferenceErrorexception.
  4. IfIsPropertyReference(V) istrue, then
    1. Let baseObj be ! ToObject(V.[[Base]]).
    2. IfIsPrivateReference(V) istrue, then
      1. Return ? PrivateGet(V.[[ReferencedName]], baseObj).
    3. Return ? baseObj.[[Get]](V.[[ReferencedName]],GetThisValue(V)).
  5. Else,
    1. Let base be V.[[Base]].
    2. Assert: base is anEnvironment Record.
    3. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see9.1).
Note

The object that may be created in step4.ais not accessible outside of the above abstract operation and theordinary object[[Get]] internal method. An implementation might choose to avoid the actual creation of the object.

6.2.4.6 PutValue ( V, W )

The abstract operation PutValue takes arguments V and W. It performs the following steps when called:

  1. ReturnIfAbrupt(V).
  2. ReturnIfAbrupt(W).
  3. If V is not aReference Record, throw aReferenceErrorexception.
  4. IfIsUnresolvableReference(V) istrue, then
    1. If V.[[Strict]] istrue, throw aReferenceErrorexception.
    2. Let globalObj beGetGlobalObject().
    3. Return ? Set(globalObj, V.[[ReferencedName]], W,false).
  5. IfIsPropertyReference(V) istrue, then
    1. Let baseObj be ! ToObject(V.[[Base]]).
    2. IfIsPrivateReference(V) istrue, then
      1. Return ? PrivateSet(V.[[ReferencedName]], baseObj, W).
    3. Let succeeded be ? baseObj.[[Set]](V.[[ReferencedName]], W,GetThisValue(V)).
    4. If succeeded isfalseand V.[[Strict]] istrue, throw aTypeErrorexception.
    5. Return.
  6. Else,
    1. Let base be V.[[Base]].
    2. Assert: base is anEnvironment Record.
    3. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see9.1).
Note

The object that may be created in step5.ais not accessible outside of the above abstract operation and theordinary object[[Set]] internal method. An implementation might choose to avoid the actual creation of that object.

6.2.4.7 GetThisValue ( V )

The abstract operation GetThisValue takes argument V. It performs the following steps when called:

  1. Assert:IsPropertyReference(V) istrue.
  2. IfIsSuperReference(V) istrue, return V.[[ThisValue]]; otherwise return V.[[Base]].

6.2.4.8 InitializeReferencedBinding ( V, W )

The abstract operation InitializeReferencedBinding takes arguments V and W. It performs the following steps when called:

  1. ReturnIfAbrupt(V).
  2. ReturnIfAbrupt(W).
  3. Assert: V is aReference Record.
  4. Assert:IsUnresolvableReference(V) isfalse.
  5. Let base be V.[[Base]].
  6. Assert: base is anEnvironment Record.
  7. Return base.InitializeBinding(V.[[ReferencedName]], W).

6.2.4.9 MakePrivateReference ( baseValue, privateIdentifier )

The abstract operation MakePrivateReference takes arguments baseValue (anECMAScript language value) and privateIdentifier (a String). It performs the following steps when called:

  1. Let privEnv be therunning execution context's PrivateEnvironment.
  2. Assert: privEnv is notnull.
  3. Let privateName be ! ResolvePrivateIdentifier(privEnv, privateIdentifier).
  4. Return theReference Record{ [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]:true, [[ThisValue]]:empty}.

6.2.5 The Property Descriptor Specification Type

The Property Descriptor type is used to explain the manipulation and reification of Object property attributes. Values of the Property Descriptor type are Records. Each field's name is an attribute name and its value is a corresponding attribute value as specified in6.1.7.1. In addition, any field may be present or absent. The schema name used within this specification to tag literal descriptions of Property Descriptor records is “PropertyDescriptor”.

Property Descriptor values may be further classified as data Property Descriptors and accessor Property Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named either [[Get]] or [[Set]]. Any Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor; however, it may be neither. A generic Property Descriptor is a Property Descriptor value that is neither a data Property Descriptor nor an accessor Property Descriptor. A fully populated Property Descriptor is one that is either an accessor Property Descriptor or a data Property Descriptor and that has all of the fields that correspond to the property attributes defined in eitherTable 3orTable 4.

The followingabstract operationsare used in this specification to operate upon Property Descriptor values:

6.2.5.1 IsAccessorDescriptor ( Desc )

The abstract operation IsAccessorDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:

  1. If Desc isundefined, returnfalse.
  2. If both Desc.[[Get]] and Desc.[[Set]] are absent, returnfalse.
  3. Returntrue.

6.2.5.2 IsDataDescriptor ( Desc )

The abstract operation IsDataDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:

  1. If Desc isundefined, returnfalse.
  2. If both Desc.[[Value]] and Desc.[[Writable]] are absent, returnfalse.
  3. Returntrue.

6.2.5.3 IsGenericDescriptor ( Desc )

The abstract operation IsGenericDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:

  1. If Desc isundefined, returnfalse.
  2. IfIsAccessorDescriptor(Desc) andIsDataDescriptor(Desc) are bothfalse, returntrue.
  3. Returnfalse.

6.2.5.4 FromPropertyDescriptor ( Desc )

The abstract operation FromPropertyDescriptor takes argument Desc (aProperty Descriptororundefined). It performs the following steps when called:

  1. If Desc isundefined, returnundefined.
  2. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  3. Assert: obj is an extensibleordinary objectwith no own properties.
  4. If Desc has a [[Value]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"value", Desc.[[Value]]).
  5. If Desc has a [[Writable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"writable", Desc.[[Writable]]).
  6. If Desc has a [[Get]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"get", Desc.[[Get]]).
  7. If Desc has a [[Set]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"set", Desc.[[Set]]).
  8. If Desc has an [[Enumerable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"enumerable", Desc.[[Enumerable]]).
  9. If Desc has a [[Configurable]] field, then
    1. Perform ! CreateDataPropertyOrThrow(obj,"configurable", Desc.[[Configurable]]).
  10. Return obj.

6.2.5.5 ToPropertyDescriptor ( Obj )

The abstract operation ToPropertyDescriptor takes argument Obj. It performs the following steps when called:

  1. IfType(Obj) is not Object, throw aTypeErrorexception.
  2. Let desc be a newProperty Descriptorthat initially has no fields.
  3. Let hasEnumerable be ? HasProperty(Obj,"enumerable").
  4. If hasEnumerable istrue, then
    1. Let enumerable be ! ToBoolean(?Get(Obj,"enumerable")).
    2. Set desc.[[Enumerable]] to enumerable.
  5. Let hasConfigurable be ? HasProperty(Obj,"configurable").
  6. If hasConfigurable istrue, then
    1. Let configurable be ! ToBoolean(?Get(Obj,"configurable")).
    2. Set desc.[[Configurable]] to configurable.
  7. Let hasValue be ? HasProperty(Obj,"value").
  8. If hasValue istrue, then
    1. Let value be ? Get(Obj,"value").
    2. Set desc.[[Value]] to value.
  9. Let hasWritable be ? HasProperty(Obj,"writable").
  10. If hasWritable istrue, then
    1. Let writable be ! ToBoolean(?Get(Obj,"writable")).
    2. Set desc.[[Writable]] to writable.
  11. Let hasGet be ? HasProperty(Obj,"get").
  12. If hasGet istrue, then
    1. Let getter be ? Get(Obj,"get").
    2. IfIsCallable(getter) isfalseand getter is notundefined, throw aTypeErrorexception.
    3. Set desc.[[Get]] to getter.
  13. Let hasSet be ? HasProperty(Obj,"set").
  14. If hasSet istrue, then
    1. Let setter be ? Get(Obj,"set").
    2. IfIsCallable(setter) isfalseand setter is notundefined, throw aTypeErrorexception.
    3. Set desc.[[Set]] to setter.
  15. If desc.[[Get]] is present or desc.[[Set]] is present, then
    1. If desc.[[Value]] is present or desc.[[Writable]] is present, throw aTypeErrorexception.
  16. Return desc.

6.2.5.6 CompletePropertyDescriptor ( Desc )

The abstract operation CompletePropertyDescriptor takes argument Desc (aProperty Descriptor). It performs the following steps when called:

  1. Assert: Desc is aProperty Descriptor.
  2. Let like be theRecord{ [[Value]]:undefined, [[Writable]]:false, [[Get]]:undefined, [[Set]]:undefined, [[Enumerable]]:false, [[Configurable]]:false}.
  3. IfIsGenericDescriptor(Desc) istrueorIsDataDescriptor(Desc) istrue, then
    1. If Desc does not have a [[Value]] field, set Desc.[[Value]] to like.[[Value]].
    2. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
  4. Else,
    1. If Desc does not have a [[Get]] field, set Desc.[[Get]] to like.[[Get]].
    2. If Desc does not have a [[Set]] field, set Desc.[[Set]] to like.[[Set]].
  5. If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
  6. If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
  7. Return Desc.

6.2.6 The Environment Record Specification Type

TheEnvironment Recordtype is used to explain the behaviour of name resolution in nested functions and blocks. This type and the operations upon it are defined in9.1.

6.2.7 The Abstract Closure Specification Type

The Abstract Closure specification type is used to refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are invoked using function application style such as closure(arg1, arg2). Likeabstract operations, invocations perform the algorithm steps described by the Abstract Closure.

In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called, each captured value is referred to by the alias that was used to capture the value.

If an Abstract Closure returns aCompletion Record, thatCompletion Record's [[Type]] must be eithernormalorthrow.

Abstract Closures are created inline as part of other algorithms, shown in the following example.

  1. Let addend be 41.
  2. Let closure be a newAbstract Closurewith parameters (x) that captures addend and performs the following steps when called:
    1. Return x + addend.
  3. Let val be closure(1).
  4. Assert: val is 42.

6.2.8 Data Blocks

The Data Block specification type is used to describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is anintegervalue in the range 0 through 255, inclusive. A Data Block value is created with a fixed number of bytes that each have the initial value 0.

For notational convenience within this specification, an array-like syntax can be used to access the individual bytes of a Data Block value. This notation presents a Data Block value as a 0-originedinteger-indexed sequence of bytes. For example, if db is a 5 byte Data Block value then db[2] can be used to access its 3rd byte.

A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has an identity (for the purposes of equality testing Shared Data Block values) that is address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the locations they contain are equal; otherwise, they are not equal and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.

The semantics of Shared Data Blocks is defined using Shared Data Block events by thememory model.Abstract operationsbelow introduce Shared Data Block events and act as the interface between evaluation semantics and the event semantics of thememory model. The events form acandidate execution, on which thememory modelacts as a filter. Please consult thememory modelfor full semantics.

Shared Data Block events are modeled by Records, defined in thememory model.

The followingabstract operationsare used in this specification to operate upon Data Block values:

6.2.8.1 CreateByteDataBlock ( size )

The abstract operation CreateByteDataBlock takes argument size (aninteger). It performs the following steps when called:

  1. Assert: size ≥ 0.
  2. Let db be a newData Blockvalue consisting of size bytes. If it is impossible to create such aData Block, throw aRangeErrorexception.
  3. Set all of the bytes of db to 0.
  4. Return db.

6.2.8.2 CreateSharedByteDataBlock ( size )

The abstract operation CreateSharedByteDataBlock takes argument size (a non-negativeinteger). It performs the following steps when called:

  1. Assert: size ≥ 0.
  2. Let db be a newShared Data Blockvalue consisting of size bytes. If it is impossible to create such aShared Data Block, throw aRangeErrorexception.
  3. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
  4. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
  5. Let zero be « 0 ».
  6. For each index i of db, do
    1. AppendWriteSharedMemory{ [[Order]]:Init, [[NoTear]]:true, [[Block]]: db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]: zero } to eventList.
  7. Return db.

6.2.8.3 CopyDataBlockBytes ( toBlock, toIndex, fromBlock, fromIndex, count )

The abstract operation CopyDataBlockBytes takes arguments toBlock, toIndex (a non-negativeinteger), fromBlock, fromIndex (a non-negativeinteger), and count (a non-negativeinteger). It performs the following steps when called:

  1. Assert: fromBlock and toBlock are distinctData BlockorShared Data Blockvalues.
  2. Let fromSize be the number of bytes in fromBlock.
  3. Assert: fromIndex + countfromSize.
  4. Let toSize be the number of bytes in toBlock.
  5. Assert: toIndex + counttoSize.
  6. Repeat, while count > 0,
    1. If fromBlock is aShared Data Block, then
      1. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
      2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
      3. Let bytes be aListwhose sole element is a nondeterministically chosenbyte value.
      4. NOTE: In implementations, bytes is the result of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
      5. Let readEvent beReadSharedMemory{ [[Order]]:Unordered, [[NoTear]]:true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
      6. Append readEvent to eventList.
      7. AppendChosen Value Record{ [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
      8. If toBlock is aShared Data Block, then
        1. AppendWriteSharedMemory{ [[Order]]:Unordered, [[NoTear]]:true, [[Block]]: toBlock, [[ByteIndex]]: toIndex, [[ElementSize]]: 1, [[Payload]]: bytes } to eventList.
      9. Else,
        1. Set toBlock[toIndex] to bytes[0].
    2. Else,
      1. Assert: toBlock is not aShared Data Block.
      2. Set toBlock[toIndex] to fromBlock[fromIndex].
    3. Set toIndex to toIndex + 1.
    4. Set fromIndex to fromIndex + 1.
    5. Set count to count - 1.
  7. ReturnNormalCompletion(empty).

6.2.9 The PrivateElement Specification Type

The PrivateElement type is aRecordused in the specification of private class fields, methods, and accessors. Although Property Descriptors are not used for private elements, private fields behave similarly to non-configurable, non-enumerable, writable data properties, private methods behave similarly to non-configurable, non-enumerable, non-writable data properties, and private accessors behave similarly to non-configurable, non-enumerable accessor properties.

Values of the PrivateElement type areRecordvalues whose fields are defined byTable 11. Such values are referred to as PrivateElements.

Table 11:PrivateElementFields
Field NameValues of the [[Kind]] field for which it is presentValueMeaning
[[Key]]AllaPrivate NameThe name of the field, method, or accessor.
[[Kind]]Allfield,method, oraccessorThe kind of the element.
[[Value]]fieldandmethodanyECMAScript language valueThe value of the field.
[[Get]]accessorFunction or UndefinedThe getter for a private accessor.
[[Set]]accessorFunction or UndefinedThe setter for a private accessor.

6.2.10 The ClassFieldDefinition Record Specification Type

The ClassFieldDefinition type is aRecordused in the specification of class fields.

Values of the ClassFieldDefinition type areRecordvalues whose fields are defined byTable 12. Such values are referred to as ClassFieldDefinition Records.

Table 12:ClassFieldDefinition RecordFields
Field NameValueMeaning
[[Name]]aPrivate Name, a String value, or a Symbol value.The name of the field.
[[Initializer]]an ECMAScriptfunction object, oremptyThe initializer of the field, if any.

6.2.11 Private Names

The Private Name specification type is used to describe a globally unique value (one which differs from any other Private Name, even if they are otherwise indistinguishable) which represents the key of a private class element (field, method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A Private Name may be installed on any ECMAScript object withPrivateFieldAddorPrivateMethodOrAccessorAdd, and then read or written usingPrivateGetandPrivateSet.

7 Abstract Operations

These operations are not a part of the ECMAScript language; they are defined here solely to aid the specification of the semantics of the ECMAScript language. Other, more specializedabstract operationsare defined throughout this specification.

7.1 Type Conversion

The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of certain constructs it is useful to define a set of conversionabstract operations. The conversionabstract operationsare polymorphic; they can accept a value of anyECMAScript language type. But no other specification types are used with these operations.

The BigInt type has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values from other types.

7.1.1 ToPrimitive ( input [ , preferredType ] )

The abstract operation ToPrimitive takes argument input and optional argument preferredType. It converts its input argument to a non-Object type. If an object is capable of converting to more than one primitive type, it may use the optional hint preferredType to favour that type. It performs the following steps when called:

  1. Assert: input is anECMAScript language value.
  2. IfType(input) is Object, then
    1. Let exoticToPrim be ? GetMethod(input,@@toPrimitive).
    2. If exoticToPrim is notundefined, then
      1. If preferredType is not present, let hint be"default".
      2. Else if preferredType isstring, let hint be"string".
      3. Else,
        1. Assert: preferredType isnumber.
        2. Let hint be"number".
      4. Let result be ? Call(exoticToPrim, input, « hint »).
      5. IfType(result) is not Object, return result.
      6. Throw aTypeErrorexception.
    3. If preferredType is not present, let preferredType benumber.
    4. Return ? OrdinaryToPrimitive(input, preferredType).
  3. Return input.
Note

When ToPrimitive is called with no hint, then it generally behaves as if the hint werenumber. However, objects may over-ride this behaviour by defining a@@toPrimitivemethod. Of the objects defined in this specification only Date objects (see21.4.4.45) and Symbol objects (see20.4.3.5) over-ride the default ToPrimitive behaviour. Date objects treat no hint as if the hint werestring.

7.1.1.1 OrdinaryToPrimitive ( O, hint )

The abstract operation OrdinaryToPrimitive takes arguments O and hint. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert: hint is eitherstringornumber.
  3. If hint isstring, then
    1. Let methodNames be «"toString","valueOf"».
  4. Else,
    1. Let methodNames be «"valueOf","toString"».
  5. For each element name of methodNames, do
    1. Let method be ? Get(O, name).
    2. IfIsCallable(method) istrue, then
      1. Let result be ? Call(method, O).
      2. IfType(result) is not Object, return result.
  6. Throw aTypeErrorexception.

7.1.2 ToBoolean ( argument )

The abstract operation ToBoolean takes argument argument. It converts argument to a value of type Boolean according toTable 13:

Table 13:ToBooleanConversions
Argument TypeResult
UndefinedReturnfalse.
NullReturnfalse.
BooleanReturn argument.
NumberIf argument is+0𝔽,-0𝔽, orNaN, returnfalse; otherwise returntrue.
StringIf argument is the empty String (its length is 0), returnfalse; otherwise returntrue.
SymbolReturntrue.
BigIntIf argument is0, returnfalse; otherwise returntrue.
ObjectReturntrue.Note

An alternate algorithm related to the [[IsHTMLDDA]] internal slot is mandated in sectionB.3.6.1.

7.1.3 ToNumeric ( value )

The abstract operation ToNumeric takes argument value. It returns value converted to a Number or a BigInt. It performs the following steps when called:

  1. Let primValue be ? ToPrimitive(value,number).
  2. IfType(primValue) is BigInt, return primValue.
  3. Return ? ToNumber(primValue).

7.1.4 ToNumber ( argument )

The abstract operation ToNumber takes argument argument. It converts argument to a value of type Number according toTable 14:

Table 14:ToNumberConversions
Argument TypeResult
UndefinedReturnNaN.
NullReturn+0𝔽.
BooleanIf argument istrue, return1𝔽. If argument isfalse, return+0𝔽.
NumberReturn argument (no conversion).
StringReturn !StringToNumber(argument).
SymbolThrow aTypeErrorexception.
BigIntThrow aTypeErrorexception.
Object

Apply the following steps:

  1. Let primValue be ? ToPrimitive(argument,number).
  2. Return ? ToNumber(primValue).

7.1.4.1 ToNumber Applied to the String Type

The abstract operationStringToNumberspecifies how to convert a String value to aNumber value, using the following grammar.

Syntax

StringNumericLiteral:::StrWhiteSpaceoptStrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceoptStrWhiteSpace:::StrWhiteSpaceCharStrWhiteSpaceoptStrWhiteSpaceChar:::WhiteSpaceLineTerminatorStrNumericLiteral:::StrDecimalLiteralNonDecimalIntegerLiteral[~Sep]StrDecimalLiteral:::StrUnsignedDecimalLiteral+StrUnsignedDecimalLiteral-StrUnsignedDecimalLiteralStrUnsignedDecimalLiteral:::InfinityDecimalDigits[~Sep].DecimalDigits[~Sep]optExponentPart[~Sep]opt.DecimalDigits[~Sep]ExponentPart[~Sep]optDecimalDigits[~Sep]ExponentPart[~Sep]opt

All grammar symbols not explicitly defined above have the definitions used in the Lexical Grammar for numeric literals (12.8.3)

Note

Some differences should be noted between the syntax of aStringNumericLiteraland aNumericLiteral:

7.1.4.1.1 StringToNumber ( str )

The abstract operation StringToNumber takes argument str (a String). It returns a Number. It performs the following steps when called:

  1. Let text be ! StringToCodePoints(str).
  2. Let literal beParseText(text,StringNumericLiteral).
  3. If literal is aListof errors, returnNaN.
  4. ReturnStringNumericValueof literal.

7.1.4.1.2 Runtime Semantics: StringNumericValue

The conversion of aStringNumericLiteralto aNumber valueis similar overall to the determination of theNumericValueof aNumericLiteral(see12.8.3), but some of the details are different.

StringNumericLiteral:::StrWhiteSpaceopt
  1. Return+0𝔽.
StringNumericLiteral:::StrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceopt
  1. ReturnStringNumericValueofStrNumericLiteral.
StrNumericLiteral:::NonDecimalIntegerLiteral
  1. Return𝔽(MV ofNonDecimalIntegerLiteral).
StrDecimalLiteral:::-StrUnsignedDecimalLiteral
  1. Let a beStringNumericValueofStrUnsignedDecimalLiteral.
  2. If a is+0𝔽, return-0𝔽.
  3. Return -a.
StrUnsignedDecimalLiteral:::Infinity
  1. Return+∞𝔽.
StrUnsignedDecimalLiteral:::DecimalDigits.DecimalDigitsoptExponentPartopt
  1. Let a be MV of the firstDecimalDigits.
  2. If the secondDecimalDigitsis present, then
    1. Let b be MV of the secondDecimalDigits.
    2. Let n be the number of code points in the secondDecimalDigits.
  3. Else,
    1. Let b be 0.
    2. Let n be 0.
  4. IfExponentPartis present, let e be MV ofExponentPart. Otherwise, let e be 0.
  5. ReturnRoundMVResult((a + (b × 10-n)) × 10e).
StrUnsignedDecimalLiteral:::.DecimalDigitsExponentPartopt
  1. Let b be MV ofDecimalDigits.
  2. IfExponentPartis present, let e be MV ofExponentPart. Otherwise, let e be 0.
  3. Let n be the number of code points inDecimalDigits.
  4. ReturnRoundMVResult(b × 10e - n).
StrUnsignedDecimalLiteral:::DecimalDigitsExponentPartopt
  1. Let a be MV ofDecimalDigits.
  2. IfExponentPartis present, let e be MV ofExponentPart. Otherwise, let e be 0.
  3. ReturnRoundMVResult(a × 10e).

7.1.4.1.3 RoundMVResult ( n )

The abstract operation RoundMVResult takes argument n (amathematical value). It converts n to a Number in animplementation-definedmanner. For the purposes of this abstract operation, a digit is significant if it is not zero or there is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this abstract operation, "themathematical valuedenoted by" a representation of amathematical valueis the inverse of "the decimal representation of" amathematical value. It performs the following steps when called:

  1. If the decimal representation of n has 20 or fewer significant digits, return𝔽(n).
  2. Let option1 be themathematical valuedenoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit.
  3. Let option2 be themathematical valuedenoted by the result of replacing each significant digit in the decimal representation of n after the 20th with a 0 digit and then incrementing it at the 20th position (with carrying as necessary).
  4. Let chosen be animplementation-definedchoice of either option1 or option2.
  5. Return𝔽(chosen).

7.1.5 ToIntegerOrInfinity ( argument )

The abstract operation ToIntegerOrInfinity takes argument argument. It converts argument to aninteger, +∞, or -∞. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽, or-0𝔽, return 0.
  3. If number is+∞𝔽, return +∞.
  4. If number is-∞𝔽, return -∞.
  5. Let integer befloor(abs((number))).
  6. If number <+0𝔽, set integer to -integer.
  7. Return integer.

7.1.6 ToInt32 ( argument )

The abstract operation ToInt32 takes argument argument. It converts argument to one of 232integral Numbervalues in the range𝔽(-231) through𝔽(231 - 1), inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int32bit be intmodulo232.
  5. If int32bit ≥ 231, return𝔽(int32bit - 232); otherwise return𝔽(int32bit).
Note

Given the above definition of ToInt32:

  • The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToInt32(ToUint32(x)) is the same value as ToInt32(x) for all values of x. (It is to preserve this latter property that+∞𝔽 and-∞𝔽 are mapped to+0𝔽.)
  • ToInt32 maps-0𝔽 to+0𝔽.

7.1.7 ToUint32 ( argument )

The abstract operation ToUint32 takes argument argument. It converts argument to one of 232integral Numbervalues in the range+0𝔽 through𝔽(232 - 1), inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int32bit be intmodulo232.
  5. Return𝔽(int32bit).
Note

Given the above definition of ToUint32:

  • Step5is the only difference between ToUint32 andToInt32.
  • The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged.
  • ToUint32(ToInt32(x)) is the same value as ToUint32(x) for all values of x. (It is to preserve this latter property that+∞𝔽 and-∞𝔽 are mapped to+0𝔽.)
  • ToUint32 maps-0𝔽 to+0𝔽.

7.1.8 ToInt16 ( argument )

The abstract operation ToInt16 takes argument argument. It converts argument to one of 216integral Numbervalues in the range𝔽(-215) through𝔽(215 - 1), inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int16bit be intmodulo216.
  5. If int16bit ≥ 215, return𝔽(int16bit - 216); otherwise return𝔽(int16bit).

7.1.9 ToUint16 ( argument )

The abstract operation ToUint16 takes argument argument. It converts argument to one of 216integral Numbervalues in the range+0𝔽 through𝔽(216 - 1), inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int16bit be intmodulo216.
  5. Return𝔽(int16bit).
Note

Given the above definition of ToUint16:

  • The substitution of 216 for 232 in step4is the only difference betweenToUint32and ToUint16.
  • ToUint16 maps-0𝔽 to+0𝔽.

7.1.10 ToInt8 ( argument )

The abstract operation ToInt8 takes argument argument. It converts argument to one of 28integral Numbervalues in the range-128𝔽 through127𝔽, inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int8bit be intmodulo28.
  5. If int8bit ≥ 27, return𝔽(int8bit - 28); otherwise return𝔽(int8bit).

7.1.11 ToUint8 ( argument )

The abstract operation ToUint8 takes argument argument. It converts argument to one of 28integral Numbervalues in the range+0𝔽 through255𝔽, inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN,+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return+0𝔽.
  3. Let int be themathematical valuewhose sign is the sign of number and whose magnitude isfloor(abs((number))).
  4. Let int8bit be intmodulo28.
  5. Return𝔽(int8bit).

7.1.12 ToUint8Clamp ( argument )

The abstract operation ToUint8Clamp takes argument argument. It converts argument to one of 28integral Numbervalues in the range+0𝔽 through255𝔽, inclusive. It performs the following steps when called:

  1. Let number be ? ToNumber(argument).
  2. If number isNaN, return+0𝔽.
  3. If(number) ≤ 0, return+0𝔽.
  4. If(number) ≥ 255, return255𝔽.
  5. Let f befloor((number)).
  6. If f + 0.5 <(number), return𝔽(f + 1).
  7. If(number) < f + 0.5, return𝔽(f).
  8. If f is odd, return𝔽(f + 1).
  9. Return𝔽(f).
Note

Unlike the other ECMAScriptintegerconversion abstract operation, ToUint8Clamp rounds rather than truncates non-integral values and does not convert+∞𝔽 to+0𝔽. ToUint8Clamp does “round half to even” tie-breaking. This differs from Math.round which does “round half up” tie-breaking.

7.1.13 ToBigInt ( argument )

The abstract operation ToBigInt takes argument argument. It converts argument to a BigInt value, or throws if an implicit conversion from Number would be required. It performs the following steps when called:

  1. Let prim be ? ToPrimitive(argument,number).
  2. Return the value that prim corresponds to inTable 15.
Table 15: BigInt Conversions
Argument TypeResult
UndefinedThrow aTypeErrorexception.
NullThrow aTypeErrorexception.
BooleanReturn 1n if prim istrueand 0n if prim isfalse.
BigIntReturn prim.
NumberThrow aTypeErrorexception.
String
  1. Let n be ! StringToBigInt(prim).
  2. If n isNaN, throw aSyntaxErrorexception.
  3. Return n.
SymbolThrow aTypeErrorexception.

7.1.14 StringToBigInt ( argument )

The abstract operation StringToBigInt takes argument argument.

Apply the algorithm in7.1.4.1with the following changes:

  • Replace theStrUnsignedDecimalLiteralproduction withDecimalDigitsto not allowInfinity, decimal points, or exponents.
  • If the MV isNaN, returnNaN, otherwise return the BigInt which exactly corresponds to the MV, rather than rounding to a Number.

7.1.15 ToBigInt64 ( argument )

The abstract operation ToBigInt64 takes argument argument. It converts argument to one of 264 BigInt values in the range(-263) through(263-1), inclusive. It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be(n)modulo264.
  3. If int64bit ≥ 263, return(int64bit - 264); otherwise return(int64bit).

7.1.16 ToBigUint64 ( argument )

The abstract operation ToBigUint64 takes argument argument. It converts argument to one of 264 BigInt values in the range0 through the BigInt value for(264-1), inclusive. It performs the following steps when called:

  1. Let n be ? ToBigInt(argument).
  2. Let int64bit be(n)modulo264.
  3. Return(int64bit).

7.1.17 ToString ( argument )

The abstract operation ToString takes argument argument. It converts argument to a value of type String according toTable 16:

Table 16:ToStringConversions
Argument TypeResult
UndefinedReturn"undefined".
NullReturn"null".
Boolean

If argument istrue, return"true".

If argument isfalse, return"false".

NumberReturn !Number::toString(argument).
StringReturn argument.
SymbolThrow aTypeErrorexception.
BigIntReturn !BigInt::toString(argument).
Object

Apply the following steps:

  1. Let primValue be ? ToPrimitive(argument,string).
  2. Return ? ToString(primValue).

7.1.18 ToObject ( argument )

The abstract operation ToObject takes argument argument. It converts argument to a value of type Object according toTable 17:

Table 17:ToObjectConversions
Argument TypeResult
UndefinedThrow aTypeErrorexception.
NullThrow aTypeErrorexception.
BooleanReturn a new Boolean object whose [[BooleanData]] internal slot is set to argument. See20.3for a description of Boolean objects.
NumberReturn a new Number object whose [[NumberData]] internal slot is set to argument. See21.1for a description of Number objects.
StringReturn a new String object whose [[StringData]] internal slot is set to argument. See22.1for a description of String objects.
SymbolReturn a new Symbol object whose [[SymbolData]] internal slot is set to argument. See20.4for a description of Symbol objects.
BigIntReturn a new BigInt object whose [[BigIntData]] internal slot is set to argument. See21.2for a description of BigInt objects.
ObjectReturn argument.

7.1.19 ToPropertyKey ( argument )

The abstract operation ToPropertyKey takes argument argument. It converts argument to a value that can be used as a property key. It performs the following steps when called:

  1. Let key be ? ToPrimitive(argument,string).
  2. IfType(key) is Symbol, then
    1. Return key.
  3. Return ! ToString(key).

7.1.20 ToLength ( argument )

The abstract operation ToLength takes argument argument. It converts argument to anintegral Numbersuitable for use as the length of anarray-like object. It performs the following steps when called:

  1. Let len be ? ToIntegerOrInfinity(argument).
  2. If len ≤ 0, return+0𝔽.
  3. Return𝔽(min(len, 253 - 1)).

7.1.21 CanonicalNumericIndexString ( argument )

The abstract operation CanonicalNumericIndexString takes argument argument. It returns argument converted to aNumber valueif it is a String representation of a Number that would be produced byToString, or the string"-0". Otherwise, it returnsundefined. It performs the following steps when called:

  1. Assert:Type(argument) is String.
  2. If argument is"-0", return-0𝔽.
  3. Let n be ! ToNumber(argument).
  4. IfSameValue(!ToString(n), argument) isfalse, returnundefined.
  5. Return n.

A canonical numeric string is any String value for which the CanonicalNumericIndexString abstract operation does not returnundefined.

7.1.22 ToIndex ( value )

The abstract operation ToIndex takes argument value. It returns value argument converted to a non-negativeintegerif it is a validinteger indexvalue. It performs the following steps when called:

  1. If value isundefined, then
    1. Return 0.
  2. Else,
    1. Let integerIndex be𝔽(?ToIntegerOrInfinity(value)).
    2. If integerIndex <+0𝔽, throw aRangeErrorexception.
    3. Let index be ! ToLength(integerIndex).
    4. If ! SameValue(integerIndex, index) isfalse, throw aRangeErrorexception.
    5. Return(index).

7.2 Testing and Comparison Operations

7.2.1 RequireObjectCoercible ( argument )

The abstract operation RequireObjectCoercible takes argument argument. It throws an error if argument is a value that cannot be converted to an Object usingToObject. It is defined byTable 18:

Table 18:RequireObjectCoercibleResults
Argument TypeResult
UndefinedThrow aTypeErrorexception.
NullThrow aTypeErrorexception.
BooleanReturn argument.
NumberReturn argument.
StringReturn argument.
SymbolReturn argument.
BigIntReturn argument.
ObjectReturn argument.

7.2.2 IsArray ( argument )

The abstract operation IsArray takes argument argument. It performs the following steps when called:

  1. IfType(argument) is not Object, returnfalse.
  2. If argument is anArray exotic object, returntrue.
  3. If argument is aProxy exotic object, then
    1. If argument.[[ProxyHandler]] isnull, throw aTypeErrorexception.
    2. Let target be argument.[[ProxyTarget]].
    3. Return ? IsArray(target).
  4. Returnfalse.

7.2.3 IsCallable ( argument )

The abstract operation IsCallable takes argument argument (anECMAScript language value). It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:

  1. IfType(argument) is not Object, returnfalse.
  2. If argument has a [[Call]] internal method, returntrue.
  3. Returnfalse.

7.2.4 IsConstructor ( argument )

The abstract operation IsConstructor takes argument argument (anECMAScript language value). It determines if argument is afunction objectwith a [[Construct]] internal method. It performs the following steps when called:

  1. IfType(argument) is not Object, returnfalse.
  2. If argument has a [[Construct]] internal method, returntrue.
  3. Returnfalse.

7.2.5 IsExtensible ( O )

The abstract operation IsExtensible takes argument O (an Object). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether additional properties can be added to O. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Return ? O.[[IsExtensible]]().

7.2.6 IsIntegralNumber ( argument )

The abstract operation IsIntegralNumber takes argument argument. It determines if argument is a finiteintegral Numbervalue. It performs the following steps when called:

  1. IfType(argument) is not Number, returnfalse.
  2. If argument isNaN,+∞𝔽, or-∞𝔽, returnfalse.
  3. Iffloor(abs((argument))) ≠abs((argument)), returnfalse.
  4. Returntrue.

7.2.7 IsPropertyKey ( argument )

The abstract operation IsPropertyKey takes argument argument (anECMAScript language value). It determines if argument is a value that may be used as a property key. It performs the following steps when called:

  1. IfType(argument) is String, returntrue.
  2. IfType(argument) is Symbol, returntrue.
  3. Returnfalse.

7.2.8 IsRegExp ( argument )

The abstract operation IsRegExp takes argument argument. It performs the following steps when called:

  1. IfType(argument) is not Object, returnfalse.
  2. Let matcher be ? Get(argument,@@match).
  3. If matcher is notundefined, return ! ToBoolean(matcher).
  4. If argument has a [[RegExpMatcher]] internal slot, returntrue.
  5. Returnfalse.

7.2.9 IsStringPrefix ( p, q )

The abstract operation IsStringPrefix takes arguments p (a String) and q (a String). It determines if p is a prefix of q. It performs the following steps when called:

  1. Assert:Type(p) is String.
  2. Assert:Type(q) is String.
  3. If q can be thestring-concatenationof p and some other String r, returntrue. Otherwise, returnfalse.
Note

Any String is a prefix of itself, because r may be the empty String.

7.2.10 SameValue ( x, y )

The abstract operation SameValue takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:

  1. IfType(x) is different fromType(y), returnfalse.
  2. IfType(x) is Number or BigInt, then
    1. Return ! Type(x)::sameValue(x, y).
  3. Return ! SameValueNonNumeric(x, y).
Note

This algorithm differs from theIsStrictlyEqualAlgorithm in its treatment of signed zeroes and NaNs.

7.2.11 SameValueZero ( x, y )

The abstract operation SameValueZero takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:

  1. IfType(x) is different fromType(y), returnfalse.
  2. IfType(x) is Number or BigInt, then
    1. Return ! Type(x)::sameValueZero(x, y).
  3. Return ! SameValueNonNumeric(x, y).
Note

SameValueZero differs fromSameValueonly in its treatment of+0𝔽 and-0𝔽.

7.2.12 SameValueNonNumeric ( x, y )

The abstract operation SameValueNonNumeric takes arguments x (anECMAScript language value) and y (anECMAScript language value). It returns a completion record whose [[Type]] isnormaland whose [[Value]] is a Boolean. It performs the following steps when called:

  1. Assert:Type(x) is not Number or BigInt.
  2. Assert:Type(x) is the same asType(y).
  3. IfType(x) is Undefined, returntrue.
  4. IfType(x) is Null, returntrue.
  5. IfType(x) is String, then
    1. If x and y are exactly the same sequence of code units (same length and same code units at corresponding indices), returntrue; otherwise, returnfalse.
  6. IfType(x) is Boolean, then
    1. If x and y are bothtrueor bothfalse, returntrue; otherwise, returnfalse.
  7. IfType(x) is Symbol, then
    1. If x and y are both the same Symbol value, returntrue; otherwise, returnfalse.
  8. If x and y are the same Object value, returntrue. Otherwise, returnfalse.

7.2.13 IsLessThan ( x, y, LeftFirst )

The abstract operation IsLessThan takes arguments x (anECMAScript language value), y (anECMAScript language value), and LeftFirst (a Boolean). It provides the semantics for the comparison x < y, returningtrue,false, orundefined(which indicates that at least one operand isNaN). The LeftFirst flag is used to control the order in which operations with potentially visible side-effects are performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If LeftFirst istrue, the x parameter corresponds to an expression that occurs to the left of the y parameter's corresponding expression. If LeftFirst isfalse, the reverse is the case and operations must be performed upon y before x. It performs the following steps when called:

  1. If the LeftFirst flag istrue, then
    1. Let px be ? ToPrimitive(x,number).
    2. Let py be ? ToPrimitive(y,number).
  2. Else,
    1. NOTE: The order of evaluation needs to be reversed to preserve left to right evaluation.
    2. Let py be ? ToPrimitive(y,number).
    3. Let px be ? ToPrimitive(x,number).
  3. IfType(px) is String andType(py) is String, then
    1. IfIsStringPrefix(py, px) istrue, returnfalse.
    2. IfIsStringPrefix(px, py) istrue, returntrue.
    3. Let k be the smallest non-negativeintegersuch that the code unit at index k within px is different from the code unit at index k within py. (There must be such a k, for neither String is a prefix of the other.)
    4. Let m be theintegerthat is the numeric value of the code unit at index k within px.
    5. Let n be theintegerthat is the numeric value of the code unit at index k within py.
    6. If m < n, returntrue. Otherwise, returnfalse.
  4. Else,
    1. IfType(px) is BigInt andType(py) is String, then
      1. Let ny be ! StringToBigInt(py).
      2. If ny isNaN, returnundefined.
      3. ReturnBigInt::lessThan(px, ny).
    2. IfType(px) is String andType(py) is BigInt, then
      1. Let nx be ! StringToBigInt(px).
      2. If nx isNaN, returnundefined.
      3. ReturnBigInt::lessThan(nx, py).
    3. NOTE: Because px and py are primitive values, evaluation order is not important.
    4. Let nx be ? ToNumeric(px).
    5. Let ny be ? ToNumeric(py).
    6. IfType(nx) is the same asType(ny), returnType(nx)::lessThan(nx, ny).
    7. Assert:Type(nx) is BigInt andType(ny) is Number, orType(nx) is Number andType(ny) is BigInt.
    8. If nx or ny isNaN, returnundefined.
    9. If nx is-∞𝔽 or ny is+∞𝔽, returntrue.
    10. If nx is+∞𝔽 or ny is-∞𝔽, returnfalse.
    11. If(nx) <(ny), returntrue; otherwise returnfalse.
Note 1

Step3differs from step2.cin the algorithm that handles the addition operator + (13.15.3) by using the logical-and operation instead of the logical-or operation.

Note 2

The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form. Also, note that for strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs from that on sequences of code point values.

7.2.14 IsLooselyEqual ( x, y )

The abstract operation IsLooselyEqual takes arguments x (anECMAScript language value) and y (anECMAScript language value). It provides the semantics for the comparison x == y, returningtrueorfalse. It performs the following steps when called:

  1. IfType(x) is the same asType(y), then
    1. ReturnIsStrictlyEqual(x, y).
  2. If x isnulland y isundefined, returntrue.
  3. If x isundefinedand y isnull, returntrue.
  4. NOTE: This step is replaced in sectionB.3.6.2.
  5. IfType(x) is Number andType(y) is String, returnIsLooselyEqual(x, ! ToNumber(y)).
  6. IfType(x) is String andType(y) is Number, returnIsLooselyEqual(!ToNumber(x), y).
  7. IfType(x) is BigInt andType(y) is String, then
    1. Let n be ! StringToBigInt(y).
    2. If n isNaN, returnfalse.
    3. ReturnIsLooselyEqual(x, n).
  8. IfType(x) is String andType(y) is BigInt, returnIsLooselyEqual(y, x).
  9. IfType(x) is Boolean, returnIsLooselyEqual(!ToNumber(x), y).
  10. IfType(y) is Boolean, returnIsLooselyEqual(x, ! ToNumber(y)).
  11. IfType(x) is either String, Number, BigInt, or Symbol andType(y) is Object, returnIsLooselyEqual(x, ? ToPrimitive(y)).
  12. IfType(x) is Object andType(y) is either String, Number, BigInt, or Symbol, returnIsLooselyEqual(?ToPrimitive(x), y).
  13. IfType(x) is BigInt andType(y) is Number, or ifType(x) is Number andType(y) is BigInt, then
    1. If x or y are any ofNaN,+∞𝔽, or-∞𝔽, returnfalse.
    2. If(x) =(y), returntrue; otherwise returnfalse.
  14. Returnfalse.

7.2.15 IsStrictlyEqual ( x, y )

The abstract operation IsStrictlyEqual takes arguments x (anECMAScript language value) and y (anECMAScript language value). It provides the semantics for the comparison x === y, returningtrueorfalse. It performs the following steps when called:

  1. IfType(x) is different fromType(y), returnfalse.
  2. IfType(x) is Number or BigInt, then
    1. Return ! Type(x)::equal(x, y).
  3. Return ! SameValueNonNumeric(x, y).
Note

This algorithm differs from theSameValueAlgorithm in its treatment of signed zeroes and NaNs.

7.3 Operations on Objects

7.3.1 MakeBasicObject ( internalSlotsList )

The abstract operation MakeBasicObject takes argument internalSlotsList. It is the source of all ECMAScript objects that are created algorithmically, including both ordinary objects and exotic objects. It factors out common steps used in creating all objects, and centralizes object creation. It performs the following steps when called:

  1. Assert: internalSlotsList is aListof internal slot names.
  2. Let obj be a newly created object with an internal slot for each name in internalSlotsList.
  3. Set obj's essential internal methods to the defaultordinary objectdefinitions specified in10.1.
  4. Assert: If the caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList contains [[Prototype]].
  5. Assert: If the caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential internal methods, then internalSlotsList contains [[Extensible]].
  6. If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] totrue.
  7. Return obj.
Note

Within this specification, exotic objects are created inabstract operationssuch asArrayCreateandBoundFunctionCreateby first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of that object's internal methods. In order to encapsulateexotic objectcreation, the object's essential internal methods are never modified outside those operations.

7.3.2 Get ( O, P )

The abstract operation Get takes arguments O (an Object) and P (a property key). It is used to retrieve the value of a specific property of an object. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Return ? O.[[Get]](P, O).

7.3.3 GetV ( V, P )

The abstract operation GetV takes arguments V (anECMAScript language value) and P (a property key). It is used to retrieve the value of a specific property of anECMAScript language value. If the value is not an object, the property lookup is performed using a wrapper object appropriate for the type of the value. It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let O be ? ToObject(V).
  3. Return ? O.[[Get]](P, V).

7.3.4 Set ( O, P, V, Throw )

The abstract operation Set takes arguments O (an Object), P (a property key), V (anECMAScript language value), and Throw (a Boolean). It is used to set the value of a specific property of an object. V is the new value for the property. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Assert:Type(Throw) is Boolean.
  4. Let success be ? O.[[Set]](P, V, O).
  5. If success isfalseand Throw istrue, throw aTypeErrorexception.
  6. Return success.

7.3.5 CreateDataProperty ( O, P, V )

The abstract operation CreateDataProperty takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.
  4. Return ? O.[[DefineOwnProperty]](P, newDesc).
Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalse.

7.3.6 CreateMethodProperty ( O, P, V )

The abstract operation CreateMethodProperty takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
  4. Return ? O.[[DefineOwnProperty]](P, newDesc).
Note

This abstract operation creates a property whose attributes are set to the same defaults used for built-in methods and methods defined using class declaration syntax. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalse.

7.3.7 CreateDataPropertyOrThrow ( O, P, V )

The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a property key), and V (anECMAScript language value). It is used to create a new own property of an object. It throws aTypeErrorexception if the requested property update cannot be performed. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let success be ? CreateDataProperty(O, P, V).
  4. If success isfalse, throw aTypeErrorexception.
  5. Return success.
Note

This abstract operation creates a property whose attributes are set to the same defaults used for properties created by the ECMAScript language assignment operator. Normally, the property will not already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will returnfalsecausing this operation to throw aTypeErrorexception.

7.3.8 DefinePropertyOrThrow ( O, P, desc )

The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a property key), and desc (aProperty Descriptor). It is used to call the [[DefineOwnProperty]] internal method of an object in a manner that will throw aTypeErrorexception if the requested property update cannot be performed. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let success be ? O.[[DefineOwnProperty]](P, desc).
  4. If success isfalse, throw aTypeErrorexception.
  5. Return success.

7.3.9 DeletePropertyOrThrow ( O, P )

The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a property key). It is used to remove a specific own property of an object. It throws an exception if the property is not configurable. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let success be ? O.[[Delete]](P).
  4. If success isfalse, throw aTypeErrorexception.
  5. Return success.

7.3.10 GetMethod ( V, P )

The abstract operation GetMethod takes arguments V (anECMAScript language value) and P (a property key). It is used to get the value of a specific property of anECMAScript language valuewhen the value of the property is expected to be a function. It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let func be ? GetV(V, P).
  3. If func is eitherundefinedornull, returnundefined.
  4. IfIsCallable(func) isfalse, throw aTypeErrorexception.
  5. Return func.

7.3.11 HasProperty ( O, P )

The abstract operation HasProperty takes arguments O (an Object) and P (a property key). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether an object has a property with the specified property key. The property may be either an own or inherited. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Return ? O.[[HasProperty]](P).

7.3.12 HasOwnProperty ( O, P )

The abstract operation HasOwnProperty takes arguments O (an Object) and P (a property key). It returns a completion record which, if its [[Type]] isnormal, has a [[Value]] which is a Boolean. It is used to determine whether an object has an own property with the specified property key. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert:IsPropertyKey(P) istrue.
  3. Let desc be ? O.[[GetOwnProperty]](P).
  4. If desc isundefined, returnfalse.
  5. Returntrue.

7.3.13 Call ( F, V [ , argumentsList ] )

The abstract operation Call takes arguments F (anECMAScript language value) and V (anECMAScript language value) and optional argument argumentsList (aListof ECMAScript language values). It is used to call the [[Call]] internal method of afunction object. F is thefunction object, V is anECMAScript language valuethat is thethisvalue of the [[Call]], and argumentsList is the value passed to the corresponding argument of the internal method. If argumentsList is not present, a new emptyListis used as its value. It performs the following steps when called:

  1. If argumentsList is not present, set argumentsList to a new emptyList.
  2. IfIsCallable(F) isfalse, throw aTypeErrorexception.
  3. Return ? F.[[Call]](V, argumentsList).

7.3.14 Construct ( F [ , argumentsList [ , newTarget ] ] )

The abstract operation Construct takes argument F (afunction object) and optional arguments argumentsList and newTarget. It is used to call the [[Construct]] internal method of afunction object. argumentsList and newTarget are the values to be passed as the corresponding arguments of the internal method. If argumentsList is not present, a new emptyListis used as its value. If newTarget is not present, F is used as its value. It performs the following steps when called:

  1. If newTarget is not present, set newTarget to F.
  2. If argumentsList is not present, set argumentsList to a new emptyList.
  3. Assert:IsConstructor(F) istrue.
  4. Assert:IsConstructor(newTarget) istrue.
  5. Return ? F.[[Construct]](argumentsList, newTarget).
Note

If newTarget is not present, this operation is equivalent to: new F(...argumentsList)

7.3.15 SetIntegrityLevel ( O, level )

The abstract operation SetIntegrityLevel takes arguments O and level. It is used to fix the set of own properties of an object. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert: level is eithersealedorfrozen.
  3. Let status be ? O.[[PreventExtensions]]().
  4. If status isfalse, returnfalse.
  5. Let keys be ? O.[[OwnPropertyKeys]]().
  6. If level issealed, then
    1. For each element k of keys, do
      1. Perform ? DefinePropertyOrThrow(O, k, PropertyDescriptor { [[Configurable]]:false}).
  7. Else,
    1. Assert: level isfrozen.
    2. For each element k of keys, do
      1. Let currentDesc be ? O.[[GetOwnProperty]](k).
      2. If currentDesc is notundefined, then
        1. IfIsAccessorDescriptor(currentDesc) istrue, then
          1. Let desc be the PropertyDescriptor { [[Configurable]]:false}.
        2. Else,
          1. Let desc be the PropertyDescriptor { [[Configurable]]:false, [[Writable]]:false}.
        3. Perform ? DefinePropertyOrThrow(O, k, desc).
  8. Returntrue.

7.3.16 TestIntegrityLevel ( O, level )

The abstract operation TestIntegrityLevel takes arguments O and level. It is used to determine if the set of own properties of an object are fixed. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Assert: level is eithersealedorfrozen.
  3. Let extensible be ? IsExtensible(O).
  4. If extensible istrue, returnfalse.
  5. NOTE: If the object is extensible, none of its properties are examined.
  6. Let keys be ? O.[[OwnPropertyKeys]]().
  7. For each element k of keys, do
    1. Let currentDesc be ? O.[[GetOwnProperty]](k).
    2. If currentDesc is notundefined, then
      1. If currentDesc.[[Configurable]] istrue, returnfalse.
      2. If level isfrozenandIsDataDescriptor(currentDesc) istrue, then
        1. If currentDesc.[[Writable]] istrue, returnfalse.
  8. Returntrue.

7.3.17 CreateArrayFromList ( elements )

The abstract operation CreateArrayFromList takes argument elements (aList). It is used to create an Array object whose elements are provided by elements. It performs the following steps when called:

  1. Assert: elements is aListwhose elements are all ECMAScript language values.
  2. Let array be ! ArrayCreate(0).
  3. Let n be 0.
  4. For each element e of elements, do
    1. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(n)), e).
    2. Set n to n + 1.
  5. Return array.

7.3.18 LengthOfArrayLike ( obj )

The abstract operation LengthOfArrayLike takes argument obj. It returns the value of the"length"property of an array-like object (as a non-negativeinteger). It performs the following steps when called:

  1. Assert:Type(obj) is Object.
  2. Return(?ToLength(?Get(obj,"length"))).

An array-like object is any object for which this operation returns anintegerrather than anabrupt completion.

Note 1
Typically, an array-like object would also have some properties withinteger indexnames. However, that is not a requirement of this definition.
Note 2
Array objects and String objects are examples of array-like objects.

7.3.19 CreateListFromArrayLike ( obj [ , elementTypes ] )

The abstract operation CreateListFromArrayLike takes argument obj and optional argument elementTypes (aListof names of ECMAScript Language Types). It is used to create aListvalue whose elements are provided by the indexed properties of obj. elementTypes contains the names of ECMAScript Language Types that are allowed for element values of theListthat is created. It performs the following steps when called:

  1. If elementTypes is not present, set elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
  2. IfType(obj) is not Object, throw aTypeErrorexception.
  3. Let len be ? LengthOfArrayLike(obj).
  4. Let list be a new emptyList.
  5. Let index be 0.
  6. Repeat, while index < len,
    1. Let indexName be ! ToString(𝔽(index)).
    2. Let next be ? Get(obj, indexName).
    3. IfType(next) is not an element of elementTypes, throw aTypeErrorexception.
    4. Append next as the last element of list.
    5. Set index to index + 1.
  7. Return list.

7.3.20 Invoke ( V, P [ , argumentsList ] )

The abstract operation Invoke takes arguments V (anECMAScript language value) and P (a property key) and optional argument argumentsList (aListof ECMAScript language values). It is used to call a method property of anECMAScript language value. V serves as both the lookup point for the property and thethisvalue of the call. argumentsList is the list of arguments values passed to the method. If argumentsList is not present, a new emptyListis used as its value. It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. If argumentsList is not present, set argumentsList to a new emptyList.
  3. Let func be ? GetV(V, P).
  4. Return ? Call(func, V, argumentsList).

7.3.21 OrdinaryHasInstance ( C, O )

The abstract operation OrdinaryHasInstance takes arguments C (anECMAScript language value) and O. It implements the default algorithm for determining if O inherits from the instance object inheritance path provided by C. It performs the following steps when called:

  1. IfIsCallable(C) isfalse, returnfalse.
  2. If C has a [[BoundTargetFunction]] internal slot, then
    1. Let BC be C.[[BoundTargetFunction]].
    2. Return ? InstanceofOperator(O, BC).
  3. IfType(O) is not Object, returnfalse.
  4. Let P be ? Get(C,"prototype").
  5. IfType(P) is not Object, throw aTypeErrorexception.
  6. Repeat,
    1. Set O to ? O.[[GetPrototypeOf]]().
    2. If O isnull, returnfalse.
    3. IfSameValue(P, O) istrue, returntrue.

7.3.22 SpeciesConstructor ( O, defaultConstructor )

The abstract operation SpeciesConstructor takes arguments O (an Object) and defaultConstructor (aconstructor). It is used to retrieve theconstructorthat should be used to create new objects that are derived from O. defaultConstructor is theconstructorto use if aconstructor@@speciesproperty cannot be found starting from O. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Let C be ? Get(O,"constructor").
  3. If C isundefined, return defaultConstructor.
  4. IfType(C) is not Object, throw aTypeErrorexception.
  5. Let S be ? Get(C,@@species).
  6. If S is eitherundefinedornull, return defaultConstructor.
  7. IfIsConstructor(S) istrue, return S.
  8. Throw aTypeErrorexception.

7.3.23 EnumerableOwnPropertyNames ( O, kind )

The abstract operation EnumerableOwnPropertyNames takes arguments O (an Object) and kind (one ofkey,value, orkey+value). It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Let ownKeys be ? O.[[OwnPropertyKeys]]().
  3. Let properties be a new emptyList.
  4. For each element key of ownKeys, do
    1. IfType(key) is String, then
      1. Let desc be ? O.[[GetOwnProperty]](key).
      2. If desc is notundefinedand desc.[[Enumerable]] istrue, then
        1. If kind iskey, append key to properties.
        2. Else,
          1. Let value be ? Get(O, key).
          2. If kind isvalue, append value to properties.
          3. Else,
            1. Assert: kind iskey+value.
            2. Let entry be ! CreateArrayFromListkey, value »).
            3. Append entry to properties.
  5. Return properties.

7.3.24 GetFunctionRealm ( obj )

The abstract operation GetFunctionRealm takes argument obj. It performs the following steps when called:

  1. Assert: ! IsCallable(obj) istrue.
  2. If obj has a [[Realm]] internal slot, then
    1. Return obj.[[Realm]].
  3. If obj is abound function exotic object, then
    1. Let target be obj.[[BoundTargetFunction]].
    2. Return ? GetFunctionRealm(target).
  4. If obj is aProxy exotic object, then
    1. If obj.[[ProxyHandler]] isnull, throw aTypeErrorexception.
    2. Let proxyTarget be obj.[[ProxyTarget]].
    3. Return ? GetFunctionRealm(proxyTarget).
  5. Returnthe current Realm Record.
Note

Step5will only be reached if obj is a non-standard functionexotic objectthat does not have a [[Realm]] internal slot.

7.3.25 CopyDataProperties ( target, source, excludedItems )

The abstract operation CopyDataProperties takes arguments target, source, and excludedItems. It performs the following steps when called:

  1. Assert:Type(target) is Object.
  2. Assert: excludedItems is aListof property keys.
  3. If source isundefinedornull, return target.
  4. Let from be ! ToObject(source).
  5. Let keys be ? from.[[OwnPropertyKeys]]().
  6. For each element nextKey of keys, do
    1. Let excluded befalse.
    2. For each element e of excludedItems, do
      1. IfSameValue(e, nextKey) istrue, then
        1. Set excluded totrue.
    3. If excluded isfalse, then
      1. Let desc be ? from.[[GetOwnProperty]](nextKey).
      2. If desc is notundefinedand desc.[[Enumerable]] istrue, then
        1. Let propValue be ? Get(from, nextKey).
        2. Perform ! CreateDataPropertyOrThrow(target, nextKey, propValue).
  7. Return target.
Note

The target passed in here is always a newly created object which is not directly accessible in case of an error being thrown.

7.3.26 PrivateElementFind ( P, O )

The abstract operation PrivateElementFind takes arguments P (aPrivate Name) and O (an Object). It performs the following steps when called:

  1. If O.[[PrivateElements]] contains aPrivateElementwhose [[Key]] is P, then
    1. Let entry be thatPrivateElement.
    2. Return entry.
  2. Returnempty.

7.3.27 PrivateFieldAdd ( P, O, value )

The abstract operation PrivateFieldAdd takes arguments P (aPrivate Name), O (an Object), and value (anECMAScript language value). It performs the following steps when called:

  1. Let entry be ! PrivateElementFind(P, O).
  2. If entry is notempty, throw aTypeErrorexception.
  3. AppendPrivateElement{ [[Key]]: P, [[Kind]]:field, [[Value]]: value } to O.[[PrivateElements]].

7.3.28 PrivateMethodOrAccessorAdd ( method, O )

The abstract operation PrivateMethodOrAccessorAdd takes arguments method (aPrivateElement) and O (an Object). It performs the following steps when called:

  1. Assert: method.[[Kind]] is eithermethodoraccessor.
  2. Let entry be ! PrivateElementFind(method.[[Key]], O).
  3. If entry is notempty, throw aTypeErrorexception.
  4. Append method to O.[[PrivateElements]].
  5. NOTE: The values for private methods and accessors are shared across instances. This step does not create a new copy of the method or accessor.

7.3.29 PrivateGet ( P, O )

The abstract operation PrivateGet takes arguments P (aPrivate Name) and O (an Object). It performs the following steps when called:

  1. Let entry be ! PrivateElementFind(P, O).
  2. If entry isempty, throw aTypeErrorexception.
  3. If entry.[[Kind]] isfieldormethod, then
    1. Return entry.[[Value]].
  4. Assert: entry.[[Kind]] isaccessor.
  5. If entry.[[Get]] isundefined, throw aTypeErrorexception.
  6. Let getter be entry.[[Get]].
  7. Return ? Call(getter, O).

7.3.30 PrivateSet ( P, O, value )

The abstract operation PrivateSet takes arguments P (aPrivate Name), O (an Object), and value (anECMAScript language value). It performs the following steps when called:

  1. Let entry be ! PrivateElementFind(P, O).
  2. If entry isempty, throw aTypeErrorexception.
  3. If entry.[[Kind]] isfield, then
    1. Set entry.[[Value]] to value.
  4. Else if entry.[[Kind]] ismethod, then
    1. Throw aTypeErrorexception.
  5. Else,
    1. Assert: entry.[[Kind]] isaccessor.
    2. If entry.[[Set]] isundefined, throw aTypeErrorexception.
    3. Let setter be entry.[[Set]].
    4. Perform ? Call(setter, O, « value »).

7.3.31 DefineField ( receiver, fieldRecord )

The abstract operation DefineField takes arguments receiver (an Object) and fieldRecord (aClassFieldDefinition Record). It performs the following steps when called:

  1. Let fieldName be fieldRecord.[[Name]].
  2. Let initializer be fieldRecord.[[Initializer]].
  3. If initializer is notempty, then
    1. Let initValue be ? Call(initializer, receiver).
  4. Else, let initValue beundefined.
  5. If fieldName is aPrivate Name, then
    1. Perform ? PrivateFieldAdd(fieldName, receiver, initValue).
  6. Else,
    1. Assert: ! IsPropertyKey(fieldName) istrue.
    2. Perform ? CreateDataPropertyOrThrow(receiver, fieldName, initValue).

7.3.32 InitializeInstanceElements ( O, constructor )

The abstract operation InitializeInstanceElements takes arguments O (an Object) and constructor (an ECMAScriptfunction object). It performs the following steps when called:

  1. Let methods be the value of constructor.[[PrivateMethods]].
  2. For eachPrivateElementmethod of methods, do
    1. Perform ? PrivateMethodOrAccessorAdd(method, O).
  3. Let fields be the value of constructor.[[Fields]].
  4. For each element fieldRecord of fields, do
    1. Perform ? DefineField(O, fieldRecord).

7.4 Operations on Iterator Objects

See Common Iteration Interfaces (27.1).

7.4.1 GetIterator ( obj [ , hint [ , method ] ] )

The abstract operation GetIterator takes argument obj and optional arguments hint and method. It performs the following steps when called:

  1. If hint is not present, set hint tosync.
  2. Assert: hint is eithersyncorasync.
  3. If method is not present, then
    1. If hint isasync, then
      1. Set method to ? GetMethod(obj,@@asyncIterator).
      2. If method isundefined, then
        1. Let syncMethod be ? GetMethod(obj,@@iterator).
        2. Let syncIteratorRecord be ? GetIterator(obj,sync, syncMethod).
        3. Return ! CreateAsyncFromSyncIterator(syncIteratorRecord).
    2. Otherwise, set method to ? GetMethod(obj,@@iterator).
  4. Let iterator be ? Call(method, obj).
  5. IfType(iterator) is not Object, throw aTypeErrorexception.
  6. Let nextMethod be ? GetV(iterator,"next").
  7. Let iteratorRecord be theRecord{ [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]:false}.
  8. Return iteratorRecord.

7.4.2 IteratorNext ( iteratorRecord [ , value ] )

The abstract operation IteratorNext takes argument iteratorRecord and optional argument value. It performs the following steps when called:

  1. If value is not present, then
    1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
  2. Else,
    1. Let result be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « value »).
  3. IfType(result) is not Object, throw aTypeErrorexception.
  4. Return result.

7.4.3 IteratorComplete ( iterResult )

The abstract operation IteratorComplete takes argument iterResult. It performs the following steps when called:

  1. Assert:Type(iterResult) is Object.
  2. Return ! ToBoolean(?Get(iterResult,"done")).

7.4.4 IteratorValue ( iterResult )

The abstract operation IteratorValue takes argument iterResult. It performs the following steps when called:

  1. Assert:Type(iterResult) is Object.
  2. Return ? Get(iterResult,"value").

7.4.5 IteratorStep ( iteratorRecord )

The abstract operation IteratorStep takes argument iteratorRecord. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns eitherfalseindicating that the iterator has reached its end or the IteratorResult object if a next value is available. It performs the following steps when called:

  1. Let result be ? IteratorNext(iteratorRecord).
  2. Let done be ? IteratorComplete(result).
  3. If done istrue, returnfalse.
  4. Return result.

7.4.6 IteratorClose ( iteratorRecord, completion )

The abstract operation IteratorClose takes arguments iteratorRecord and completion. It is used to notify an iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert:Type(iteratorRecord.[[Iterator]]) is Object.
  2. Assert: completion is aCompletion Record.
  3. Let iterator be iteratorRecord.[[Iterator]].
  4. Let innerResult beGetMethod(iterator,"return").
  5. If innerResult.[[Type]] isnormal, then
    1. Let return be innerResult.[[Value]].
    2. If return isundefined, returnCompletion(completion).
    3. Set innerResult toCall(return, iterator).
  6. If completion.[[Type]] isthrow, returnCompletion(completion).
  7. If innerResult.[[Type]] isthrow, returnCompletion(innerResult).
  8. IfType(innerResult.[[Value]]) is not Object, throw aTypeErrorexception.
  9. ReturnCompletion(completion).

7.4.7 AsyncIteratorClose ( iteratorRecord, completion )

The abstract operation AsyncIteratorClose takes arguments iteratorRecord and completion. It is used to notify an async iterator that it should perform any actions it would normally perform when it has reached its completed state. It performs the following steps when called:

  1. Assert:Type(iteratorRecord.[[Iterator]]) is Object.
  2. Assert: completion is aCompletion Record.
  3. Let iterator be iteratorRecord.[[Iterator]].
  4. Let innerResult beGetMethod(iterator,"return").
  5. If innerResult.[[Type]] isnormal, then
    1. Let return be innerResult.[[Value]].
    2. If return isundefined, returnCompletion(completion).
    3. Set innerResult toCall(return, iterator).
    4. If innerResult.[[Type]] isnormal, set innerResult toAwait(innerResult.[[Value]]).
  6. If completion.[[Type]] isthrow, returnCompletion(completion).
  7. If innerResult.[[Type]] isthrow, returnCompletion(innerResult).
  8. IfType(innerResult.[[Value]]) is not Object, throw aTypeErrorexception.
  9. ReturnCompletion(completion).

7.4.8 CreateIterResultObject ( value, done )

The abstract operation CreateIterResultObject takes arguments value and done. It creates an object that supports the IteratorResult interface. It performs the following steps when called:

  1. Assert:Type(done) is Boolean.
  2. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ! CreateDataPropertyOrThrow(obj,"value", value).
  4. Perform ! CreateDataPropertyOrThrow(obj,"done", done).
  5. Return obj.

7.4.9 CreateListIteratorRecord ( list )

The abstract operation CreateListIteratorRecord takes argument list. It creates an Iterator (27.1.1.2) object record whose next method returns the successive elements of list. It performs the following steps when called:

  1. Let closure be a newAbstract Closurewith no parameters that captures list and performs the following steps when called:
    1. For each element E of list, do
      1. Perform ? Yield(E).
    2. Returnundefined.
  2. Let iterator be ! CreateIteratorFromClosure(closure,empty,%IteratorPrototype%).
  3. ReturnRecord{ [[Iterator]]: iterator, [[NextMethod]]: %GeneratorFunction.prototype.prototype.next%, [[Done]]:false}.
Note

The list iterator object is never directly accessible to ECMAScript code.

7.4.10 IterableToList ( items [ , method ] )

The abstract operation IterableToList takes argument items and optional argument method. It performs the following steps when called:

  1. If method is present, then
    1. Let iteratorRecord be ? GetIterator(items,sync, method).
  2. Else,
    1. Let iteratorRecord be ? GetIterator(items,sync).
  3. Let values be a new emptyList.
  4. Let next betrue.
  5. Repeat, while next is notfalse,
    1. Set next to ? IteratorStep(iteratorRecord).
    2. If next is notfalse, then
      1. Let nextValue be ? IteratorValue(next).
      2. Append nextValue to the end of theListvalues.
  6. Return values.

8 Syntax-Directed Operations

In addition to those defined in this section, specialized syntax-directed operations are defined throughout this specification.

8.1 Scope Analysis

8.1.1 Static Semantics: BoundNames

Note

"*default*"is used within this specification as a synthetic name for hoistable anonymous functions that are defined using export declarations.

BindingIdentifier:Identifier
  1. Return aListwhose sole element is theStringValueofIdentifier.
BindingIdentifier:yield
  1. Return aListwhose sole element is"yield".
BindingIdentifier:await
  1. Return aListwhose sole element is"await".
LexicalDeclaration:LetOrConstBindingList;
  1. Return theBoundNamesofBindingList.
BindingList:BindingList,LexicalBinding
  1. Let names1 be theBoundNamesofBindingList.
  2. Let names2 be theBoundNamesofLexicalBinding.
  3. Return thelist-concatenationof names1 and names2.
LexicalBinding:BindingIdentifierInitializeropt
  1. Return theBoundNamesofBindingIdentifier.
LexicalBinding:BindingPatternInitializer
  1. Return theBoundNamesofBindingPattern.
VariableDeclarationList:VariableDeclarationList,VariableDeclaration
  1. Let names1 beBoundNamesofVariableDeclarationList.
  2. Let names2 beBoundNamesofVariableDeclaration.
  3. Return thelist-concatenationof names1 and names2.
VariableDeclaration:BindingIdentifierInitializeropt
  1. Return theBoundNamesofBindingIdentifier.
VariableDeclaration:BindingPatternInitializer
  1. Return theBoundNamesofBindingPattern.
ObjectBindingPattern:{}
  1. Return a new emptyList.
ObjectBindingPattern:{BindingPropertyList,BindingRestProperty}
  1. Let names1 beBoundNamesofBindingPropertyList.
  2. Let names2 beBoundNamesofBindingRestProperty.
  3. Return thelist-concatenationof names1 and names2.
ArrayBindingPattern:[Elisionopt]
  1. Return a new emptyList.
ArrayBindingPattern:[ElisionoptBindingRestElement]
  1. Return theBoundNamesofBindingRestElement.
ArrayBindingPattern:[BindingElementList,Elisionopt]
  1. Return theBoundNamesofBindingElementList.
ArrayBindingPattern:[BindingElementList,ElisionoptBindingRestElement]
  1. Let names1 beBoundNamesofBindingElementList.
  2. Let names2 beBoundNamesofBindingRestElement.
  3. Return thelist-concatenationof names1 and names2.
BindingPropertyList:BindingPropertyList,BindingProperty
  1. Let names1 beBoundNamesofBindingPropertyList.
  2. Let names2 beBoundNamesofBindingProperty.
  3. Return thelist-concatenationof names1 and names2.
BindingElementList:BindingElementList,BindingElisionElement
  1. Let names1 beBoundNamesofBindingElementList.
  2. Let names2 beBoundNamesofBindingElisionElement.
  3. Return thelist-concatenationof names1 and names2.
BindingElisionElement:ElisionoptBindingElement
  1. ReturnBoundNamesofBindingElement.
BindingProperty:PropertyName:BindingElement
  1. Return theBoundNamesofBindingElement.
SingleNameBinding:BindingIdentifierInitializeropt
  1. Return theBoundNamesofBindingIdentifier.
BindingElement:BindingPatternInitializeropt
  1. Return theBoundNamesofBindingPattern.
ForDeclaration:LetOrConstForBinding
  1. Return theBoundNamesofForBinding.
FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}
  1. Return theBoundNamesofBindingIdentifier.
FunctionDeclaration:function(FormalParameters){FunctionBody}
  1. Return «"*default*"».
FormalParameters:[empty]
  1. Return a new emptyList.
FormalParameters:FormalParameterList,FunctionRestParameter
  1. Let names1 beBoundNamesofFormalParameterList.
  2. Let names2 beBoundNamesofFunctionRestParameter.
  3. Return thelist-concatenationof names1 and names2.
FormalParameterList:FormalParameterList,FormalParameter
  1. Let names1 beBoundNamesofFormalParameterList.
  2. Let names2 beBoundNamesofFormalParameter.
  3. Return thelist-concatenationof names1 and names2.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. Return theBoundNamesof formals.
GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}
  1. Return theBoundNamesofBindingIdentifier.
GeneratorDeclaration:function*(FormalParameters){GeneratorBody}
  1. Return «"*default*"».
AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}
  1. Return theBoundNamesofBindingIdentifier.
AsyncGeneratorDeclaration:asyncfunction*(FormalParameters){AsyncGeneratorBody}
  1. Return «"*default*"».
ClassDeclaration:classBindingIdentifierClassTail
  1. Return theBoundNamesofBindingIdentifier.
ClassDeclaration:classClassTail
  1. Return «"*default*"».
AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}
  1. Return theBoundNamesofBindingIdentifier.
AsyncFunctionDeclaration:asyncfunction(FormalParameters){AsyncFunctionBody}
  1. Return «"*default*"».
CoverCallExpressionAndAsyncArrowHead:MemberExpressionArguments
  1. Let head be theAsyncArrowHeadthat iscoveredbyCoverCallExpressionAndAsyncArrowHead.
  2. Return theBoundNamesof head.
ImportDeclaration:importImportClauseFromClause;
  1. Return theBoundNamesofImportClause.
ImportDeclaration:importModuleSpecifier;
  1. Return a new emptyList.
ImportClause:ImportedDefaultBinding,NameSpaceImport
  1. Let names1 be theBoundNamesofImportedDefaultBinding.
  2. Let names2 be theBoundNamesofNameSpaceImport.
  3. Return thelist-concatenationof names1 and names2.
ImportClause:ImportedDefaultBinding,NamedImports
  1. Let names1 be theBoundNamesofImportedDefaultBinding.
  2. Let names2 be theBoundNamesofNamedImports.
  3. Return thelist-concatenationof names1 and names2.
NamedImports:{}
  1. Return a new emptyList.
ImportsList:ImportsList,ImportSpecifier
  1. Let names1 be theBoundNamesofImportsList.
  2. Let names2 be theBoundNamesofImportSpecifier.
  3. Return thelist-concatenationof names1 and names2.
ImportSpecifier:IdentifierNameasImportedBinding
  1. Return theBoundNamesofImportedBinding.
ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;
  1. Return a new emptyList.
ExportDeclaration:exportVariableStatement
  1. Return theBoundNamesofVariableStatement.
ExportDeclaration:exportDeclaration
  1. Return theBoundNamesofDeclaration.
ExportDeclaration:exportdefaultHoistableDeclaration
  1. Let declarationNames be theBoundNamesofHoistableDeclaration.
  2. If declarationNames does not include the element"*default*", append"*default*"to declarationNames.
  3. Return declarationNames.
ExportDeclaration:exportdefaultClassDeclaration
  1. Let declarationNames be theBoundNamesofClassDeclaration.
  2. If declarationNames does not include the element"*default*", append"*default*"to declarationNames.
  3. Return declarationNames.
ExportDeclaration:exportdefaultAssignmentExpression;
  1. Return «"*default*"».

8.1.2 Static Semantics: DeclarationPart

HoistableDeclaration:FunctionDeclaration
  1. ReturnFunctionDeclaration.
HoistableDeclaration:GeneratorDeclaration
  1. ReturnGeneratorDeclaration.
HoistableDeclaration:AsyncFunctionDeclaration
  1. ReturnAsyncFunctionDeclaration.
HoistableDeclaration:AsyncGeneratorDeclaration
  1. ReturnAsyncGeneratorDeclaration.
Declaration:ClassDeclaration
  1. ReturnClassDeclaration.
Declaration:LexicalDeclaration
  1. ReturnLexicalDeclaration.

8.1.3 Static Semantics: IsConstantDeclaration

LexicalDeclaration:LetOrConstBindingList;
  1. ReturnIsConstantDeclarationofLetOrConst.
LetOrConst:let
  1. Returnfalse.
LetOrConst:const
  1. Returntrue.
FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}function(FormalParameters){FunctionBody}GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}function*(FormalParameters){GeneratorBody}AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}asyncfunction*(FormalParameters){AsyncGeneratorBody}AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}asyncfunction(FormalParameters){AsyncFunctionBody}
  1. Returnfalse.
ClassDeclaration:classBindingIdentifierClassTailclassClassTail
  1. Returnfalse.
ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;exportdefaultAssignmentExpression;
  1. Returnfalse.
Note

It is not necessary to treat export defaultAssignmentExpressionas a constant declaration because there is no syntax that permits assignment to the internal bound name used to reference a module's default object.

8.1.4 Static Semantics: LexicallyDeclaredNames

Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let names1 beLexicallyDeclaredNamesofStatementList.
  2. Let names2 beLexicallyDeclaredNamesofStatementListItem.
  3. Return thelist-concatenationof names1 and names2.
StatementListItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnLexicallyDeclaredNamesofLabelledStatement.
  2. Return a new emptyList.
StatementListItem:Declaration
  1. Return theBoundNamesofDeclaration.
CaseBlock:{}
  1. Return a new emptyList.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, let names1 be theLexicallyDeclaredNamesof the firstCaseClauses.
  2. Else, let names1 be a new emptyList.
  3. Let names2 beLexicallyDeclaredNamesofDefaultClause.
  4. If the secondCaseClausesis present, let names3 be theLexicallyDeclaredNamesof the secondCaseClauses.
  5. Else, let names3 be a new emptyList.
  6. Return thelist-concatenationof names1, names2, and names3.
CaseClauses:CaseClausesCaseClause
  1. Let names1 beLexicallyDeclaredNamesofCaseClauses.
  2. Let names2 beLexicallyDeclaredNamesofCaseClause.
  3. Return thelist-concatenationof names1 and names2.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, return theLexicallyDeclaredNamesofStatementList.
  2. Return a new emptyList.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, return theLexicallyDeclaredNamesofStatementList.
  2. Return a new emptyList.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theLexicallyDeclaredNamesofLabelledItem.
LabelledItem:Statement
  1. Return a new emptyList.
LabelledItem:FunctionDeclaration
  1. ReturnBoundNamesofFunctionDeclaration.
FunctionStatementList:[empty]
  1. Return a new emptyList.
FunctionStatementList:StatementList
  1. ReturnTopLevelLexicallyDeclaredNamesofStatementList.
ConciseBody:ExpressionBody
  1. Return a new emptyList.
AsyncConciseBody:ExpressionBody
  1. Return a new emptyList.
ScriptBody:StatementList
  1. ReturnTopLevelLexicallyDeclaredNamesofStatementList.
Note 1

At the top level of aScript, function declarations are treated like var declarations rather than like lexical declarations.

Note 2

The LexicallyDeclaredNames of aModuleincludes the names of all of its imported bindings.

ModuleItemList:ModuleItemListModuleItem
  1. Let names1 beLexicallyDeclaredNamesofModuleItemList.
  2. Let names2 beLexicallyDeclaredNamesofModuleItem.
  3. Return thelist-concatenationof names1 and names2.
ModuleItem:ImportDeclaration
  1. Return theBoundNamesofImportDeclaration.
ModuleItem:ExportDeclaration
  1. IfExportDeclarationis exportVariableStatement, return a new emptyList.
  2. Return theBoundNamesofExportDeclaration.
ModuleItem:StatementListItem
  1. ReturnLexicallyDeclaredNamesofStatementListItem.
Note 3

At the top level of aModule, function declarations are treated like lexical declarations rather than like var declarations.

8.1.5 Static Semantics: LexicallyScopedDeclarations

StatementList:StatementListStatementListItem
  1. Let declarations1 beLexicallyScopedDeclarationsofStatementList.
  2. Let declarations2 beLexicallyScopedDeclarationsofStatementListItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
StatementListItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnLexicallyScopedDeclarationsofLabelledStatement.
  2. Return a new emptyList.
StatementListItem:Declaration
  1. Return aListwhose sole element isDeclarationPartofDeclaration.
CaseBlock:{}
  1. Return a new emptyList.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, let declarations1 be theLexicallyScopedDeclarationsof the firstCaseClauses.
  2. Else, let declarations1 be a new emptyList.
  3. Let declarations2 beLexicallyScopedDeclarationsofDefaultClause.
  4. If the secondCaseClausesis present, let declarations3 be theLexicallyScopedDeclarationsof the secondCaseClauses.
  5. Else, let declarations3 be a new emptyList.
  6. Return thelist-concatenationof declarations1, declarations2, and declarations3.
CaseClauses:CaseClausesCaseClause
  1. Let declarations1 beLexicallyScopedDeclarationsofCaseClauses.
  2. Let declarations2 beLexicallyScopedDeclarationsofCaseClause.
  3. Return thelist-concatenationof declarations1 and declarations2.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, return theLexicallyScopedDeclarationsofStatementList.
  2. Return a new emptyList.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, return theLexicallyScopedDeclarationsofStatementList.
  2. Return a new emptyList.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theLexicallyScopedDeclarationsofLabelledItem.
LabelledItem:Statement
  1. Return a new emptyList.
LabelledItem:FunctionDeclaration
  1. Return aListwhose sole element isFunctionDeclaration.
FunctionStatementList:[empty]
  1. Return a new emptyList.
FunctionStatementList:StatementList
  1. Return theTopLevelLexicallyScopedDeclarationsofStatementList.
ConciseBody:ExpressionBody
  1. Return a new emptyList.
AsyncConciseBody:ExpressionBody
  1. Return a new emptyList.
ScriptBody:StatementList
  1. ReturnTopLevelLexicallyScopedDeclarationsofStatementList.
Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItemListModuleItem
  1. Let declarations1 beLexicallyScopedDeclarationsofModuleItemList.
  2. Let declarations2 beLexicallyScopedDeclarationsofModuleItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
ModuleItem:ImportDeclaration
  1. Return a new emptyList.
ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;exportVariableStatement
  1. Return a new emptyList.
ExportDeclaration:exportDeclaration
  1. Return aListwhose sole element isDeclarationPartofDeclaration.
ExportDeclaration:exportdefaultHoistableDeclaration
  1. Return aListwhose sole element isDeclarationPartofHoistableDeclaration.
ExportDeclaration:exportdefaultClassDeclaration
  1. Return aListwhose sole element isClassDeclaration.
ExportDeclaration:exportdefaultAssignmentExpression;
  1. Return aListwhose sole element is thisExportDeclaration.

8.1.6 Static Semantics: VarDeclaredNames

Statement:EmptyStatementExpressionStatementContinueStatementBreakStatementReturnStatementThrowStatementDebuggerStatement
  1. Return a new emptyList.
Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let names1 beVarDeclaredNamesofStatementList.
  2. Let names2 beVarDeclaredNamesofStatementListItem.
  3. Return thelist-concatenationof names1 and names2.
StatementListItem:Declaration
  1. Return a new emptyList.
VariableStatement:varVariableDeclarationList;
  1. ReturnBoundNamesofVariableDeclarationList.
IfStatement:if(Expression)StatementelseStatement
  1. Let names1 beVarDeclaredNamesof the firstStatement.
  2. Let names2 beVarDeclaredNamesof the secondStatement.
  3. Return thelist-concatenationof names1 and names2.
IfStatement:if(Expression)Statement
  1. Return theVarDeclaredNamesofStatement.
DoWhileStatement:doStatementwhile(Expression);
  1. Return theVarDeclaredNamesofStatement.
WhileStatement:while(Expression)Statement
  1. Return theVarDeclaredNamesofStatement.
ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statement
  1. Return theVarDeclaredNamesofStatement.
ForStatement:for(varVariableDeclarationList;Expressionopt;Expressionopt)Statement
  1. Let names1 beBoundNamesofVariableDeclarationList.
  2. Let names2 beVarDeclaredNamesofStatement.
  3. Return thelist-concatenationof names1 and names2.
ForStatement:for(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. Return theVarDeclaredNamesofStatement.
ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement
  1. Return theVarDeclaredNamesofStatement.
ForInOfStatement:for(varForBindinginExpression)Statementfor(varForBindingofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statement
  1. Let names1 be theBoundNamesofForBinding.
  2. Let names2 be theVarDeclaredNamesofStatement.
  3. Return thelist-concatenationof names1 and names2.
Note

This section is extended by AnnexB.3.5.

WithStatement:with(Expression)Statement
  1. Return theVarDeclaredNamesofStatement.
SwitchStatement:switch(Expression)CaseBlock
  1. Return theVarDeclaredNamesofCaseBlock.
CaseBlock:{}
  1. Return a new emptyList.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, let names1 be theVarDeclaredNamesof the firstCaseClauses.
  2. Else, let names1 be a new emptyList.
  3. Let names2 beVarDeclaredNamesofDefaultClause.
  4. If the secondCaseClausesis present, let names3 be theVarDeclaredNamesof the secondCaseClauses.
  5. Else, let names3 be a new emptyList.
  6. Return thelist-concatenationof names1, names2, and names3.
CaseClauses:CaseClausesCaseClause
  1. Let names1 beVarDeclaredNamesofCaseClauses.
  2. Let names2 beVarDeclaredNamesofCaseClause.
  3. Return thelist-concatenationof names1 and names2.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, return theVarDeclaredNamesofStatementList.
  2. Return a new emptyList.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, return theVarDeclaredNamesofStatementList.
  2. Return a new emptyList.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theVarDeclaredNamesofLabelledItem.
LabelledItem:FunctionDeclaration
  1. Return a new emptyList.
TryStatement:tryBlockCatch
  1. Let names1 beVarDeclaredNamesofBlock.
  2. Let names2 beVarDeclaredNamesofCatch.
  3. Return thelist-concatenationof names1 and names2.
TryStatement:tryBlockFinally
  1. Let names1 beVarDeclaredNamesofBlock.
  2. Let names2 beVarDeclaredNamesofFinally.
  3. Return thelist-concatenationof names1 and names2.
TryStatement:tryBlockCatchFinally
  1. Let names1 beVarDeclaredNamesofBlock.
  2. Let names2 beVarDeclaredNamesofCatch.
  3. Let names3 beVarDeclaredNamesofFinally.
  4. Return thelist-concatenationof names1, names2, and names3.
Catch:catch(CatchParameter)Block
  1. Return theVarDeclaredNamesofBlock.
FunctionStatementList:[empty]
  1. Return a new emptyList.
FunctionStatementList:StatementList
  1. ReturnTopLevelVarDeclaredNamesofStatementList.
ConciseBody:ExpressionBody
  1. Return a new emptyList.
AsyncConciseBody:ExpressionBody
  1. Return a new emptyList.
ScriptBody:StatementList
  1. ReturnTopLevelVarDeclaredNamesofStatementList.
Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItemListModuleItem
  1. Let names1 beVarDeclaredNamesofModuleItemList.
  2. Let names2 beVarDeclaredNamesofModuleItem.
  3. Return thelist-concatenationof names1 and names2.
ModuleItem:ImportDeclaration
  1. Return a new emptyList.
ModuleItem:ExportDeclaration
  1. IfExportDeclarationis exportVariableStatement, returnBoundNamesofExportDeclaration.
  2. Return a new emptyList.

8.1.7 Static Semantics: VarScopedDeclarations

Statement:EmptyStatementExpressionStatementContinueStatementBreakStatementReturnStatementThrowStatementDebuggerStatement
  1. Return a new emptyList.
Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let declarations1 beVarScopedDeclarationsofStatementList.
  2. Let declarations2 beVarScopedDeclarationsofStatementListItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
StatementListItem:Declaration
  1. Return a new emptyList.
VariableDeclarationList:VariableDeclaration
  1. Return aListwhose sole element isVariableDeclaration.
VariableDeclarationList:VariableDeclarationList,VariableDeclaration
  1. Let declarations1 beVarScopedDeclarationsofVariableDeclarationList.
  2. Return thelist-concatenationof declarations1 and «VariableDeclaration».
IfStatement:if(Expression)StatementelseStatement
  1. Let declarations1 beVarScopedDeclarationsof the firstStatement.
  2. Let declarations2 beVarScopedDeclarationsof the secondStatement.
  3. Return thelist-concatenationof declarations1 and declarations2.
IfStatement:if(Expression)Statement
  1. Return theVarScopedDeclarationsofStatement.
DoWhileStatement:doStatementwhile(Expression);
  1. Return theVarScopedDeclarationsofStatement.
WhileStatement:while(Expression)Statement
  1. Return theVarScopedDeclarationsofStatement.
ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statement
  1. Return theVarScopedDeclarationsofStatement.
ForStatement:for(varVariableDeclarationList;Expressionopt;Expressionopt)Statement
  1. Let declarations1 beVarScopedDeclarationsofVariableDeclarationList.
  2. Let declarations2 beVarScopedDeclarationsofStatement.
  3. Return thelist-concatenationof declarations1 and declarations2.
ForStatement:for(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. Return theVarScopedDeclarationsofStatement.
ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement
  1. Return theVarScopedDeclarationsofStatement.
ForInOfStatement:for(varForBindinginExpression)Statementfor(varForBindingofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statement
  1. Let declarations1 be aListwhose sole element isForBinding.
  2. Let declarations2 beVarScopedDeclarationsofStatement.
  3. Return thelist-concatenationof declarations1 and declarations2.
Note

This section is extended by AnnexB.3.5.

WithStatement:with(Expression)Statement
  1. Return theVarScopedDeclarationsofStatement.
SwitchStatement:switch(Expression)CaseBlock
  1. Return theVarScopedDeclarationsofCaseBlock.
CaseBlock:{}
  1. Return a new emptyList.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, let declarations1 be theVarScopedDeclarationsof the firstCaseClauses.
  2. Else, let declarations1 be a new emptyList.
  3. Let declarations2 beVarScopedDeclarationsofDefaultClause.
  4. If the secondCaseClausesis present, let declarations3 be theVarScopedDeclarationsof the secondCaseClauses.
  5. Else, let declarations3 be a new emptyList.
  6. Return thelist-concatenationof declarations1, declarations2, and declarations3.
CaseClauses:CaseClausesCaseClause
  1. Let declarations1 beVarScopedDeclarationsofCaseClauses.
  2. Let declarations2 beVarScopedDeclarationsofCaseClause.
  3. Return thelist-concatenationof declarations1 and declarations2.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, return theVarScopedDeclarationsofStatementList.
  2. Return a new emptyList.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, return theVarScopedDeclarationsofStatementList.
  2. Return a new emptyList.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theVarScopedDeclarationsofLabelledItem.
LabelledItem:FunctionDeclaration
  1. Return a new emptyList.
TryStatement:tryBlockCatch
  1. Let declarations1 beVarScopedDeclarationsofBlock.
  2. Let declarations2 beVarScopedDeclarationsofCatch.
  3. Return thelist-concatenationof declarations1 and declarations2.
TryStatement:tryBlockFinally
  1. Let declarations1 beVarScopedDeclarationsofBlock.
  2. Let declarations2 beVarScopedDeclarationsofFinally.
  3. Return thelist-concatenationof declarations1 and declarations2.
TryStatement:tryBlockCatchFinally
  1. Let declarations1 beVarScopedDeclarationsofBlock.
  2. Let declarations2 beVarScopedDeclarationsofCatch.
  3. Let declarations3 beVarScopedDeclarationsofFinally.
  4. Return thelist-concatenationof declarations1, declarations2, and declarations3.
Catch:catch(CatchParameter)Block
  1. Return theVarScopedDeclarationsofBlock.
FunctionStatementList:[empty]
  1. Return a new emptyList.
FunctionStatementList:StatementList
  1. Return theTopLevelVarScopedDeclarationsofStatementList.
ConciseBody:ExpressionBody
  1. Return a new emptyList.
AsyncConciseBody:ExpressionBody
  1. Return a new emptyList.
ScriptBody:StatementList
  1. ReturnTopLevelVarScopedDeclarationsofStatementList.
Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItemListModuleItem
  1. Let declarations1 beVarScopedDeclarationsofModuleItemList.
  2. Let declarations2 beVarScopedDeclarationsofModuleItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
ModuleItem:ImportDeclaration
  1. Return a new emptyList.
ModuleItem:ExportDeclaration
  1. IfExportDeclarationis exportVariableStatement, returnVarScopedDeclarationsofVariableStatement.
  2. Return a new emptyList.

8.1.8 Static Semantics: TopLevelLexicallyDeclaredNames

StatementList:StatementListStatementListItem
  1. Let names1 beTopLevelLexicallyDeclaredNamesofStatementList.
  2. Let names2 beTopLevelLexicallyDeclaredNamesofStatementListItem.
  3. Return thelist-concatenationof names1 and names2.
StatementListItem:Statement
  1. Return a new emptyList.
StatementListItem:Declaration
  1. IfDeclarationisDeclaration:HoistableDeclaration, then
    1. Return « ».
  2. Return theBoundNamesofDeclaration.
Note

At the top level of a function, or script, function declarations are treated like var declarations rather than like lexical declarations.

LabelledStatement:LabelIdentifier:LabelledItem
  1. Return a new emptyList.

8.1.9 Static Semantics: TopLevelLexicallyScopedDeclarations

Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let declarations1 beTopLevelLexicallyScopedDeclarationsofStatementList.
  2. Let declarations2 beTopLevelLexicallyScopedDeclarationsofStatementListItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
StatementListItem:Statement
  1. Return a new emptyList.
StatementListItem:Declaration
  1. IfDeclarationisDeclaration:HoistableDeclaration, then
    1. Return « ».
  2. Return aListwhose sole element isDeclaration.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return a new emptyList.

8.1.10 Static Semantics: TopLevelVarDeclaredNames

Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let names1 beTopLevelVarDeclaredNamesofStatementList.
  2. Let names2 beTopLevelVarDeclaredNamesofStatementListItem.
  3. Return thelist-concatenationof names1 and names2.
StatementListItem:Declaration
  1. IfDeclarationisDeclaration:HoistableDeclaration, then
    1. Return theBoundNamesofHoistableDeclaration.
  2. Return a new emptyList.
StatementListItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnTopLevelVarDeclaredNamesofStatement.
  2. ReturnVarDeclaredNamesofStatement.
Note

At the top level of a function or script, inner function declarations are treated like var declarations.

LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theTopLevelVarDeclaredNamesofLabelledItem.
LabelledItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnTopLevelVarDeclaredNamesofStatement.
  2. ReturnVarDeclaredNamesofStatement.
LabelledItem:FunctionDeclaration
  1. ReturnBoundNamesofFunctionDeclaration.

8.1.11 Static Semantics: TopLevelVarScopedDeclarations

Block:{}
  1. Return a new emptyList.
StatementList:StatementListStatementListItem
  1. Let declarations1 beTopLevelVarScopedDeclarationsofStatementList.
  2. Let declarations2 beTopLevelVarScopedDeclarationsofStatementListItem.
  3. Return thelist-concatenationof declarations1 and declarations2.
StatementListItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnTopLevelVarScopedDeclarationsofStatement.
  2. ReturnVarScopedDeclarationsofStatement.
StatementListItem:Declaration
  1. IfDeclarationisDeclaration:HoistableDeclaration, then
    1. Let declaration beDeclarationPartofHoistableDeclaration.
    2. Return « declaration ».
  2. Return a new emptyList.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Return theTopLevelVarScopedDeclarationsofLabelledItem.
LabelledItem:Statement
  1. IfStatementisStatement:LabelledStatement, returnTopLevelVarScopedDeclarationsofStatement.
  2. ReturnVarScopedDeclarationsofStatement.
LabelledItem:FunctionDeclaration
  1. Return aListwhose sole element isFunctionDeclaration.

8.2 Labels

8.2.1 Static Semantics: ContainsDuplicateLabels

With parameter labelSet.

Statement:VariableStatementEmptyStatementExpressionStatementContinueStatementBreakStatementReturnStatementThrowStatementDebuggerStatementBlock:{}StatementListItem:Declaration
  1. Returnfalse.
StatementList:StatementListStatementListItem
  1. Let hasDuplicates beContainsDuplicateLabelsofStatementListwith argument labelSet.
  2. If hasDuplicates istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofStatementListItemwith argument labelSet.
IfStatement:if(Expression)StatementelseStatement
  1. Let hasDuplicate beContainsDuplicateLabelsof the firstStatementwith argument labelSet.
  2. If hasDuplicate istrue, returntrue.
  3. ReturnContainsDuplicateLabelsof the secondStatementwith argument labelSet.
IfStatement:if(Expression)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
DoWhileStatement:doStatementwhile(Expression);
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
WhileStatement:while(Expression)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statementfor(varVariableDeclarationList;Expressionopt;Expressionopt)Statementfor(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(varForBindinginExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(varForBindingofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
Note

This section is extended by AnnexB.3.5.

WithStatement:with(Expression)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.
SwitchStatement:switch(Expression)CaseBlock
  1. ReturnContainsDuplicateLabelsofCaseBlockwith argument labelSet.
CaseBlock:{}
  1. Returnfalse.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, then
    1. IfContainsDuplicateLabelsof the firstCaseClauseswith argument labelSet istrue, returntrue.
  2. IfContainsDuplicateLabelsofDefaultClausewith argument labelSet istrue, returntrue.
  3. If the secondCaseClausesis not present, returnfalse.
  4. ReturnContainsDuplicateLabelsof the secondCaseClauseswith argument labelSet.
CaseClauses:CaseClausesCaseClause
  1. Let hasDuplicates beContainsDuplicateLabelsofCaseClauseswith argument labelSet.
  2. If hasDuplicates istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofCaseClausewith argument labelSet.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, returnContainsDuplicateLabelsofStatementListwith argument labelSet.
  2. Returnfalse.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, returnContainsDuplicateLabelsofStatementListwith argument labelSet.
  2. Returnfalse.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Let label be theStringValueofLabelIdentifier.
  2. If label is an element of labelSet, returntrue.
  3. Let newLabelSet be thelist-concatenationof labelSet and « label ».
  4. ReturnContainsDuplicateLabelsofLabelledItemwith argument newLabelSet.
LabelledItem:FunctionDeclaration
  1. Returnfalse.
TryStatement:tryBlockCatch
  1. Let hasDuplicates beContainsDuplicateLabelsofBlockwith argument labelSet.
  2. If hasDuplicates istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofCatchwith argument labelSet.
TryStatement:tryBlockFinally
  1. Let hasDuplicates beContainsDuplicateLabelsofBlockwith argument labelSet.
  2. If hasDuplicates istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofFinallywith argument labelSet.
TryStatement:tryBlockCatchFinally
  1. IfContainsDuplicateLabelsofBlockwith argument labelSet istrue, returntrue.
  2. IfContainsDuplicateLabelsofCatchwith argument labelSet istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofFinallywith argument labelSet.
Catch:catch(CatchParameter)Block
  1. ReturnContainsDuplicateLabelsofBlockwith argument labelSet.
FunctionStatementList:[empty]
  1. Returnfalse.
ModuleItemList:ModuleItemListModuleItem
  1. Let hasDuplicates beContainsDuplicateLabelsofModuleItemListwith argument labelSet.
  2. If hasDuplicates istrue, returntrue.
  3. ReturnContainsDuplicateLabelsofModuleItemwith argument labelSet.
ModuleItem:ImportDeclarationExportDeclaration
  1. Returnfalse.

8.2.2 Static Semantics: ContainsUndefinedBreakTarget

With parameter labelSet.

Statement:VariableStatementEmptyStatementExpressionStatementContinueStatementReturnStatementThrowStatementDebuggerStatementBlock:{}StatementListItem:Declaration
  1. Returnfalse.
StatementList:StatementListStatementListItem
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetofStatementListwith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofStatementListItemwith argument labelSet.
IfStatement:if(Expression)StatementelseStatement
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetof the firstStatementwith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetof the secondStatementwith argument labelSet.
IfStatement:if(Expression)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
DoWhileStatement:doStatementwhile(Expression);
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
WhileStatement:while(Expression)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statementfor(varVariableDeclarationList;Expressionopt;Expressionopt)Statementfor(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(varForBindinginExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(varForBindingofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
Note

This section is extended by AnnexB.3.5.

BreakStatement:break;
  1. Returnfalse.
BreakStatement:breakLabelIdentifier;
  1. If theStringValueofLabelIdentifieris not an element of labelSet, returntrue.
  2. Returnfalse.
WithStatement:with(Expression)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.
SwitchStatement:switch(Expression)CaseBlock
  1. ReturnContainsUndefinedBreakTargetofCaseBlockwith argument labelSet.
CaseBlock:{}
  1. Returnfalse.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, then
    1. IfContainsUndefinedBreakTargetof the firstCaseClauseswith argument labelSet istrue, returntrue.
  2. IfContainsUndefinedBreakTargetofDefaultClausewith argument labelSet istrue, returntrue.
  3. If the secondCaseClausesis not present, returnfalse.
  4. ReturnContainsUndefinedBreakTargetof the secondCaseClauseswith argument labelSet.
CaseClauses:CaseClausesCaseClause
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetofCaseClauseswith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofCaseClausewith argument labelSet.
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, returnContainsUndefinedBreakTargetofStatementListwith argument labelSet.
  2. Returnfalse.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, returnContainsUndefinedBreakTargetofStatementListwith argument labelSet.
  2. Returnfalse.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Let label be theStringValueofLabelIdentifier.
  2. Let newLabelSet be thelist-concatenationof labelSet and « label ».
  3. ReturnContainsUndefinedBreakTargetofLabelledItemwith argument newLabelSet.
LabelledItem:FunctionDeclaration
  1. Returnfalse.
TryStatement:tryBlockCatch
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetofBlockwith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofCatchwith argument labelSet.
TryStatement:tryBlockFinally
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetofBlockwith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofFinallywith argument labelSet.
TryStatement:tryBlockCatchFinally
  1. IfContainsUndefinedBreakTargetofBlockwith argument labelSet istrue, returntrue.
  2. IfContainsUndefinedBreakTargetofCatchwith argument labelSet istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofFinallywith argument labelSet.
Catch:catch(CatchParameter)Block
  1. ReturnContainsUndefinedBreakTargetofBlockwith argument labelSet.
FunctionStatementList:[empty]
  1. Returnfalse.
ModuleItemList:ModuleItemListModuleItem
  1. Let hasUndefinedLabels beContainsUndefinedBreakTargetofModuleItemListwith argument labelSet.
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedBreakTargetofModuleItemwith argument labelSet.
ModuleItem:ImportDeclarationExportDeclaration
  1. Returnfalse.

8.2.3 Static Semantics: ContainsUndefinedContinueTarget

With parameters iterationSet and labelSet.

Statement:VariableStatementEmptyStatementExpressionStatementBreakStatementReturnStatementThrowStatementDebuggerStatementBlock:{}StatementListItem:Declaration
  1. Returnfalse.
BreakableStatement:IterationStatement
  1. Let newIterationSet be thelist-concatenationof iterationSet and labelSet.
  2. ReturnContainsUndefinedContinueTargetofIterationStatementwith arguments newIterationSet and « ».
StatementList:StatementListStatementListItem
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetofStatementListwith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofStatementListItemwith arguments iterationSet and « ».
IfStatement:if(Expression)StatementelseStatement
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetof the firstStatementwith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetof the secondStatementwith arguments iterationSet and « ».
IfStatement:if(Expression)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
DoWhileStatement:doStatementwhile(Expression);
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
WhileStatement:while(Expression)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statementfor(varVariableDeclarationList;Expressionopt;Expressionopt)Statementfor(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(varForBindinginExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(varForBindingofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
Note

This section is extended by AnnexB.3.5.

ContinueStatement:continue;
  1. Returnfalse.
ContinueStatement:continueLabelIdentifier;
  1. If theStringValueofLabelIdentifieris not an element of iterationSet, returntrue.
  2. Returnfalse.
WithStatement:with(Expression)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».
SwitchStatement:switch(Expression)CaseBlock
  1. ReturnContainsUndefinedContinueTargetofCaseBlockwith arguments iterationSet and « ».
CaseBlock:{}
  1. Returnfalse.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. If the firstCaseClausesis present, then
    1. IfContainsUndefinedContinueTargetof the firstCaseClauseswith arguments iterationSet and « » istrue, returntrue.
  2. IfContainsUndefinedContinueTargetofDefaultClausewith arguments iterationSet and « » istrue, returntrue.
  3. If the secondCaseClausesis not present, returnfalse.
  4. ReturnContainsUndefinedContinueTargetof the secondCaseClauseswith arguments iterationSet and « ».
CaseClauses:CaseClausesCaseClause
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetofCaseClauseswith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofCaseClausewith arguments iterationSet and « ».
CaseClause:caseExpression:StatementListopt
  1. If theStatementListis present, returnContainsUndefinedContinueTargetofStatementListwith arguments iterationSet and « ».
  2. Returnfalse.
DefaultClause:default:StatementListopt
  1. If theStatementListis present, returnContainsUndefinedContinueTargetofStatementListwith arguments iterationSet and « ».
  2. Returnfalse.
LabelledStatement:LabelIdentifier:LabelledItem
  1. Let label be theStringValueofLabelIdentifier.
  2. Let newLabelSet be thelist-concatenationof labelSet and « label ».
  3. ReturnContainsUndefinedContinueTargetofLabelledItemwith arguments iterationSet and newLabelSet.
LabelledItem:FunctionDeclaration
  1. Returnfalse.
TryStatement:tryBlockCatch
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetofBlockwith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofCatchwith arguments iterationSet and « ».
TryStatement:tryBlockFinally
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetofBlockwith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofFinallywith arguments iterationSet and « ».
TryStatement:tryBlockCatchFinally
  1. IfContainsUndefinedContinueTargetofBlockwith arguments iterationSet and « » istrue, returntrue.
  2. IfContainsUndefinedContinueTargetofCatchwith arguments iterationSet and « » istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofFinallywith arguments iterationSet and « ».
Catch:catch(CatchParameter)Block
  1. ReturnContainsUndefinedContinueTargetofBlockwith arguments iterationSet and « ».
FunctionStatementList:[empty]
  1. Returnfalse.
ModuleItemList:ModuleItemListModuleItem
  1. Let hasUndefinedLabels beContainsUndefinedContinueTargetofModuleItemListwith arguments iterationSet and « ».
  2. If hasUndefinedLabels istrue, returntrue.
  3. ReturnContainsUndefinedContinueTargetofModuleItemwith arguments iterationSet and « ».
ModuleItem:ImportDeclarationExportDeclaration
  1. Returnfalse.

8.3 Function Name Inference

8.3.1 Static Semantics: HasName

PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. IfIsFunctionDefinitionof expr isfalse, returnfalse.
  3. ReturnHasNameof expr.
FunctionExpression:function(FormalParameters){FunctionBody}GeneratorExpression:function*(FormalParameters){GeneratorBody}AsyncGeneratorExpression:asyncfunction*(FormalParameters){AsyncGeneratorBody}AsyncFunctionExpression:asyncfunction(FormalParameters){AsyncFunctionBody}ArrowFunction:ArrowParameters=>ConciseBodyAsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBodyCoverCallExpressionAndAsyncArrowHead=>AsyncConciseBodyClassExpression:classClassTail
  1. Returnfalse.
FunctionExpression:functionBindingIdentifier(FormalParameters){FunctionBody}GeneratorExpression:function*BindingIdentifier(FormalParameters){GeneratorBody}AsyncGeneratorExpression:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}AsyncFunctionExpression:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}ClassExpression:classBindingIdentifierClassTail
  1. Returntrue.

8.3.2 Static Semantics: IsFunctionDefinition

PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnIsFunctionDefinitionof expr.
PrimaryExpression:thisIdentifierReferenceLiteralArrayLiteralObjectLiteralRegularExpressionLiteralTemplateLiteralMemberExpression:MemberExpression[Expression]MemberExpression.IdentifierNameMemberExpressionTemplateLiteralSuperPropertyMetaPropertynewMemberExpressionArgumentsMemberExpression.PrivateIdentifierNewExpression:newNewExpressionLeftHandSideExpression:CallExpressionOptionalExpressionUpdateExpression:LeftHandSideExpression++LeftHandSideExpression--++UnaryExpression--UnaryExpressionUnaryExpression:deleteUnaryExpressionvoidUnaryExpressiontypeofUnaryExpression+UnaryExpression-UnaryExpression~UnaryExpression!UnaryExpressionAwaitExpressionExponentiationExpression:UpdateExpression**ExponentiationExpressionMultiplicativeExpression:MultiplicativeExpressionMultiplicativeOperatorExponentiationExpressionAdditiveExpression:AdditiveExpression+MultiplicativeExpressionAdditiveExpression-MultiplicativeExpressionShiftExpression:ShiftExpression<<AdditiveExpressionShiftExpression>>AdditiveExpressionShiftExpression>>>AdditiveExpressionRelationalExpression:RelationalExpression<ShiftExpressionRelationalExpression>ShiftExpressionRelationalExpression<=ShiftExpressionRelationalExpression>=ShiftExpressionRelationalExpressioninstanceofShiftExpressionRelationalExpressioninShiftExpressionPrivateIdentifierinShiftExpressionEqualityExpression:EqualityExpression==RelationalExpressionEqualityExpression!=RelationalExpressionEqualityExpression===RelationalExpressionEqualityExpression!==RelationalExpressionBitwiseANDExpression:BitwiseANDExpression&EqualityExpressionBitwiseXORExpression:BitwiseXORExpression^BitwiseANDExpressionBitwiseORExpression:BitwiseORExpression|BitwiseXORExpressionLogicalANDExpression:LogicalANDExpression&&BitwiseORExpressionLogicalORExpression:LogicalORExpression||LogicalANDExpressionCoalesceExpression:CoalesceExpressionHead??BitwiseORExpressionConditionalExpression:ShortCircuitExpression?AssignmentExpression:AssignmentExpressionAssignmentExpression:YieldExpressionLeftHandSideExpression=AssignmentExpressionLeftHandSideExpressionAssignmentOperatorAssignmentExpressionLeftHandSideExpression&&=AssignmentExpressionLeftHandSideExpression||=AssignmentExpressionLeftHandSideExpression??=AssignmentExpressionExpression:Expression,AssignmentExpression
  1. Returnfalse.
AssignmentExpression:ArrowFunctionAsyncArrowFunctionFunctionExpression:functionBindingIdentifieropt(FormalParameters){FunctionBody}GeneratorExpression:function*BindingIdentifieropt(FormalParameters){GeneratorBody}AsyncGeneratorExpression:asyncfunction*BindingIdentifieropt(FormalParameters){AsyncGeneratorBody}AsyncFunctionExpression:asyncfunctionBindingIdentifieropt(FormalParameters){AsyncFunctionBody}ClassExpression:classBindingIdentifieroptClassTail
  1. Returntrue.

8.3.3 Static Semantics: IsAnonymousFunctionDefinition ( expr )

The abstract operation IsAnonymousFunctionDefinition takes argument expr (aParse NodeforAssignmentExpressionor aParse NodeforInitializer). It determines if its argument is a function definition that does not bind a name. It performs the following steps when called:

  1. IfIsFunctionDefinitionof expr isfalse, returnfalse.
  2. Let hasName beHasNameof expr.
  3. If hasName istrue, returnfalse.
  4. Returntrue.

8.3.4 Static Semantics: IsIdentifierRef

PrimaryExpression:IdentifierReference
  1. Returntrue.
PrimaryExpression:thisLiteralArrayLiteralObjectLiteralFunctionExpressionClassExpressionGeneratorExpressionAsyncFunctionExpressionAsyncGeneratorExpressionRegularExpressionLiteralTemplateLiteralCoverParenthesizedExpressionAndArrowParameterListMemberExpression:MemberExpression[Expression]MemberExpression.IdentifierNameMemberExpressionTemplateLiteralSuperPropertyMetaPropertynewMemberExpressionArgumentsMemberExpression.PrivateIdentifierNewExpression:newNewExpressionLeftHandSideExpression:CallExpressionOptionalExpression
  1. Returnfalse.

8.3.5 Runtime Semantics: NamedEvaluation

With parameter name.

PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. Return the result of performingNamedEvaluationfor expr with argument name.
ParenthesizedExpression:(Expression)
  1. Assert:IsAnonymousFunctionDefinition(Expression) istrue.
  2. Return the result of performingNamedEvaluationforExpressionwith argument name.
FunctionExpression:function(FormalParameters){FunctionBody}
  1. ReturnInstantiateOrdinaryFunctionExpressionofFunctionExpressionwith argument name.
GeneratorExpression:function*(FormalParameters){GeneratorBody}
  1. ReturnInstantiateGeneratorFunctionExpressionofGeneratorExpressionwith argument name.
AsyncGeneratorExpression:asyncfunction*(FormalParameters){AsyncGeneratorBody}
  1. ReturnInstantiateAsyncGeneratorFunctionExpressionofAsyncGeneratorExpressionwith argument name.
AsyncFunctionExpression:asyncfunction(FormalParameters){AsyncFunctionBody}
  1. ReturnInstantiateAsyncFunctionExpressionofAsyncFunctionExpressionwith argument name.
ArrowFunction:ArrowParameters=>ConciseBody
  1. ReturnInstantiateArrowFunctionExpressionofArrowFunctionwith argument name.
AsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBodyCoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody
  1. ReturnInstantiateAsyncArrowFunctionExpressionofAsyncArrowFunctionwith argument name.
ClassExpression:classClassTail
  1. Let value be the result ofClassDefinitionEvaluationofClassTailwith argumentsundefinedand name.
  2. ReturnIfAbrupt(value).
  3. Set value.[[SourceText]] to the source text matched byClassExpression.
  4. Return value.

8.4 Contains

8.4.1 Static Semantics: Contains

With parameter symbol.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of Contains:

  1. For each child node child of thisParse Node, do
    1. If child is an instance of symbol, returntrue.
    2. If child is an instance of a nonterminal, then
      1. Let contained be the result of childContainssymbol.
      2. If contained istrue, returntrue.
  2. Returnfalse.
FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}function(FormalParameters){FunctionBody}FunctionExpression:functionBindingIdentifieropt(FormalParameters){FunctionBody}GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}function*(FormalParameters){GeneratorBody}GeneratorExpression:function*BindingIdentifieropt(FormalParameters){GeneratorBody}AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}asyncfunction*(FormalParameters){AsyncGeneratorBody}AsyncGeneratorExpression:asyncfunction*BindingIdentifieropt(FormalParameters){AsyncGeneratorBody}AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}asyncfunction(FormalParameters){AsyncFunctionBody}AsyncFunctionExpression:asyncfunctionBindingIdentifieropt(FormalParameters){AsyncFunctionBody}
  1. Returnfalse.
Note 1

Static semantic rules that depend upon substructure generally do not look into function definitions.

ClassTail:ClassHeritageopt{ClassBody}
  1. If symbol isClassBody, returntrue.
  2. If symbol isClassHeritage, then
    1. IfClassHeritageis present, returntrue; otherwise returnfalse.
  3. IfClassHeritageis present, then
    1. IfClassHeritageContainssymbol istrue, returntrue.
  4. Return the result ofComputedPropertyContainsforClassBodywith argument symbol.
Note 2

Static semantic rules that depend upon substructure generally do not look into class bodies except forPropertyNames.

ArrowFunction:ArrowParameters=>ConciseBody
  1. If symbol is not one ofNewTarget,SuperProperty,SuperCall, super or this, returnfalse.
  2. IfArrowParametersContainssymbol istrue, returntrue.
  3. ReturnConciseBodyContainssymbol.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. Return formalsContainssymbol.
AsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBody
  1. If symbol is not one ofNewTarget,SuperProperty,SuperCall, super, or this, returnfalse.
  2. ReturnAsyncConciseBodyContainssymbol.
AsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody
  1. If symbol is not one ofNewTarget,SuperProperty,SuperCall, super, or this, returnfalse.
  2. Let head be theAsyncArrowHeadthat iscoveredbyCoverCallExpressionAndAsyncArrowHead.
  3. If headContainssymbol istrue, returntrue.
  4. ReturnAsyncConciseBodyContainssymbol.
Note 3

Contains is used to detect new.target, this, and super usage within anArrowFunctionorAsyncArrowFunction.

PropertyDefinition:MethodDefinition
  1. If symbol isMethodDefinition, returntrue.
  2. Return the result ofComputedPropertyContainsforMethodDefinitionwith argument symbol.
LiteralPropertyName:IdentifierName
  1. Returnfalse.
MemberExpression:MemberExpression.IdentifierName
  1. IfMemberExpressionContainssymbol istrue, returntrue.
  2. Returnfalse.
SuperProperty:super.IdentifierName
  1. If symbol is theReservedWordsuper, returntrue.
  2. Returnfalse.
CallExpression:CallExpression.IdentifierName
  1. IfCallExpressionContainssymbol istrue, returntrue.
  2. Returnfalse.
OptionalChain:?.IdentifierName
  1. Returnfalse.
OptionalChain:OptionalChain.IdentifierName
  1. IfOptionalChainContainssymbol istrue, returntrue.
  2. Returnfalse.

8.4.2 Static Semantics: ComputedPropertyContains

With parameter symbol.

ClassElementName:PrivateIdentifierPropertyName:LiteralPropertyName
  1. Returnfalse.
PropertyName:ComputedPropertyName
  1. Return the result ofComputedPropertyNameContainssymbol.
MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}getClassElementName(){FunctionBody}setClassElementName(PropertySetParameterList){FunctionBody}
  1. Return the result ofComputedPropertyContainsforClassElementNamewith argument symbol.
GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}
  1. Return the result ofComputedPropertyContainsforClassElementNamewith argument symbol.
AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}
  1. Return the result ofComputedPropertyContainsforClassElementNamewith argument symbol.
ClassElementList:ClassElementListClassElement
  1. Let inList beComputedPropertyContainsofClassElementListwith argument symbol.
  2. If inList istrue, returntrue.
  3. Return the result ofComputedPropertyContainsforClassElementwith argument symbol.
ClassElement:;
  1. Returnfalse.
AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}
  1. Return the result ofComputedPropertyContainsforClassElementNamewith argument symbol.
FieldDefinition:ClassElementNameInitializeropt
  1. Return the result ofComputedPropertyContainsforClassElementNamewith argument symbol.

8.5 Miscellaneous

These operations are used in multiple places throughout the specification.

8.5.1 Runtime Semantics: InstantiateFunctionObject

With parameters scope and privateScope.

FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}function(FormalParameters){FunctionBody}
  1. Return ?InstantiateOrdinaryFunctionObjectofFunctionDeclarationwith arguments scope and privateScope.
GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}function*(FormalParameters){GeneratorBody}
  1. Return ?InstantiateGeneratorFunctionObjectofGeneratorDeclarationwith arguments scope and privateScope.
AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}asyncfunction*(FormalParameters){AsyncGeneratorBody}
  1. Return ?InstantiateAsyncGeneratorFunctionObjectofAsyncGeneratorDeclarationwith arguments scope and privateScope.
AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}asyncfunction(FormalParameters){AsyncFunctionBody}
  1. Return ?InstantiateAsyncFunctionObjectofAsyncFunctionDeclarationwith arguments scope and privateScope.

8.5.2 Runtime Semantics: BindingInitialization

With parameters value and environment.

Note

undefinedis passed for environment to indicate that aPutValueoperation should be used to assign the initialization value. This is the case for var statements and formal parameter lists of some non-strict functions (See10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

BindingIdentifier:Identifier
  1. Let name beStringValueofIdentifier.
  2. Return ? InitializeBoundName(name, value, environment).
BindingIdentifier:yield
  1. Return ? InitializeBoundName("yield", value, environment).
BindingIdentifier:await
  1. Return ? InitializeBoundName("await", value, environment).
BindingPattern:ObjectBindingPattern
  1. Perform ? RequireObjectCoercible(value).
  2. Return the result of performingBindingInitializationforObjectBindingPatternusing value and environment as arguments.
BindingPattern:ArrayBindingPattern
  1. Let iteratorRecord be ? GetIterator(value).
  2. Let result beIteratorBindingInitializationofArrayBindingPatternwith arguments iteratorRecord and environment.
  3. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, result).
  4. Return result.
ObjectBindingPattern:{}
  1. ReturnNormalCompletion(empty).
ObjectBindingPattern:{BindingPropertyList}{BindingPropertyList,}
  1. Perform ?PropertyBindingInitializationforBindingPropertyListusing value and environment as the arguments.
  2. ReturnNormalCompletion(empty).
ObjectBindingPattern:{BindingRestProperty}
  1. Let excludedNames be a new emptyList.
  2. Return the result of performingRestBindingInitializationofBindingRestPropertywith value, environment, and excludedNames as the arguments.
ObjectBindingPattern:{BindingPropertyList,BindingRestProperty}
  1. Let excludedNames be ?PropertyBindingInitializationofBindingPropertyListwith arguments value and environment.
  2. Return the result of performingRestBindingInitializationofBindingRestPropertywith arguments value, environment, and excludedNames.

8.5.2.1 InitializeBoundName ( name, value, environment )

The abstract operation InitializeBoundName takes arguments name, value, and environment. It performs the following steps when called:

  1. Assert:Type(name) is String.
  2. If environment is notundefined, then
    1. Perform environment.InitializeBinding(name, value).
    2. ReturnNormalCompletion(undefined).
  3. Else,
    1. Let lhs beResolveBinding(name).
    2. Return ? PutValue(lhs, value).

8.5.3 Runtime Semantics: IteratorBindingInitialization

With parameters iteratorRecord and environment.

Note

Whenundefinedis passed for environment it indicates that aPutValueoperation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

ArrayBindingPattern:[]
  1. ReturnNormalCompletion(empty).
ArrayBindingPattern:[Elision]
  1. Return the result of performingIteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
ArrayBindingPattern:[ElisionoptBindingRestElement]
  1. IfElisionis present, then
    1. Perform ?IteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
  2. Return the result of performingIteratorBindingInitializationforBindingRestElementwith iteratorRecord and environment as arguments.
ArrayBindingPattern:[BindingElementList,Elision]
  1. Perform ?IteratorBindingInitializationforBindingElementListwith iteratorRecord and environment as arguments.
  2. Return the result of performingIteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
ArrayBindingPattern:[BindingElementList,ElisionoptBindingRestElement]
  1. Perform ?IteratorBindingInitializationforBindingElementListwith iteratorRecord and environment as arguments.
  2. IfElisionis present, then
    1. Perform ?IteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
  3. Return the result of performingIteratorBindingInitializationforBindingRestElementwith iteratorRecord and environment as arguments.
BindingElementList:BindingElementList,BindingElisionElement
  1. Perform ?IteratorBindingInitializationforBindingElementListwith iteratorRecord and environment as arguments.
  2. Return the result of performingIteratorBindingInitializationforBindingElisionElementusing iteratorRecord and environment as arguments.
BindingElisionElement:ElisionBindingElement
  1. Perform ?IteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
  2. Return the result of performingIteratorBindingInitializationofBindingElementwith iteratorRecord and environment as the arguments.
SingleNameBinding:BindingIdentifierInitializeropt
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. If iteratorRecord.[[Done]] isfalse, then
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    5. Else,
      1. Let v beIteratorValue(next).
      2. If v is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(v).
  4. If iteratorRecord.[[Done]] istrue, let v beundefined.
  5. IfInitializeris present and v isundefined, then
    1. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
      1. Set v to the result of performingNamedEvaluationforInitializerwith argument bindingId.
    2. Else,
      1. Let defaultValue be the result of evaluatingInitializer.
      2. Set v to ? GetValue(defaultValue).
  6. If environment isundefined, return ? PutValue(lhs, v).
  7. ReturnInitializeReferencedBinding(lhs, v).
BindingElement:BindingPatternInitializeropt
  1. If iteratorRecord.[[Done]] isfalse, then
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    5. Else,
      1. Let v beIteratorValue(next).
      2. If v is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(v).
  2. If iteratorRecord.[[Done]] istrue, let v beundefined.
  3. IfInitializeris present and v isundefined, then
    1. Let defaultValue be the result of evaluatingInitializer.
    2. Set v to ? GetValue(defaultValue).
  4. Return the result of performingBindingInitializationofBindingPatternwith v and environment as the arguments.
BindingRestElement:...BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValueofBindingIdentifier, environment).
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat,
    1. If iteratorRecord.[[Done]] isfalse, then
      1. Let next beIteratorStep(iteratorRecord).
      2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(next).
      4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    2. If iteratorRecord.[[Done]] istrue, then
      1. If environment isundefined, return ? PutValue(lhs, A).
      2. ReturnInitializeReferencedBinding(lhs, A).
    3. Let nextValue beIteratorValue(next).
    4. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n + 1.
BindingRestElement:...BindingPattern
  1. Let A be ! ArrayCreate(0).
  2. Let n be 0.
  3. Repeat,
    1. If iteratorRecord.[[Done]] isfalse, then
      1. Let next beIteratorStep(iteratorRecord).
      2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(next).
      4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    2. If iteratorRecord.[[Done]] istrue, then
      1. Return the result of performingBindingInitializationofBindingPatternwith A and environment as the arguments.
    3. Let nextValue beIteratorValue(next).
    4. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    5. ReturnIfAbrupt(nextValue).
    6. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
    7. Set n to n + 1.
FormalParameters:[empty]
  1. ReturnNormalCompletion(empty).
FormalParameters:FormalParameterList,FunctionRestParameter
  1. Perform ?IteratorBindingInitializationforFormalParameterListusing iteratorRecord and environment as the arguments.
  2. Return the result of performingIteratorBindingInitializationforFunctionRestParameterusing iteratorRecord and environment as the arguments.
FormalParameterList:FormalParameterList,FormalParameter
  1. Perform ?IteratorBindingInitializationforFormalParameterListusing iteratorRecord and environment as the arguments.
  2. Return the result of performingIteratorBindingInitializationforFormalParameterusing iteratorRecord and environment as the arguments.
ArrowParameters:BindingIdentifier
  1. Assert: iteratorRecord.[[Done]] isfalse.
  2. Let next beIteratorStep(iteratorRecord).
  3. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
  4. ReturnIfAbrupt(next).
  5. If next isfalse, set iteratorRecord.[[Done]] totrue.
  6. Else,
    1. Let v beIteratorValue(next).
    2. If v is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(v).
  7. If iteratorRecord.[[Done]] istrue, let v beundefined.
  8. Return the result of performingBindingInitializationforBindingIdentifierusing v and environment as the arguments.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnIteratorBindingInitializationof formals with arguments iteratorRecord and environment.
AsyncArrowBindingIdentifier:BindingIdentifier
  1. Assert: iteratorRecord.[[Done]] isfalse.
  2. Let next beIteratorStep(iteratorRecord).
  3. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
  4. ReturnIfAbrupt(next).
  5. If next isfalse, set iteratorRecord.[[Done]] totrue.
  6. Else,
    1. Let v beIteratorValue(next).
    2. If v is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(v).
  7. If iteratorRecord.[[Done]] istrue, let v beundefined.
  8. Return the result of performingBindingInitializationforBindingIdentifierusing v and environment as the arguments.

8.5.4 Static Semantics: AssignmentTargetType

IdentifierReference:Identifier
  1. If thisIdentifierReferenceis contained instrict mode codeandStringValueofIdentifieris"eval"or"arguments", returninvalid.
  2. Returnsimple.
IdentifierReference:yieldawaitCallExpression:CallExpression[Expression]CallExpression.IdentifierNameCallExpression.PrivateIdentifierMemberExpression:MemberExpression[Expression]MemberExpression.IdentifierNameSuperPropertyMemberExpression.PrivateIdentifier
  1. Returnsimple.
PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnAssignmentTargetTypeof expr.
PrimaryExpression:thisLiteralArrayLiteralObjectLiteralFunctionExpressionClassExpressionGeneratorExpressionAsyncFunctionExpressionAsyncGeneratorExpressionRegularExpressionLiteralTemplateLiteralCallExpression:CoverCallExpressionAndAsyncArrowHeadSuperCallImportCallCallExpressionArgumentsCallExpressionTemplateLiteralNewExpression:newNewExpressionMemberExpression:MemberExpressionTemplateLiteralnewMemberExpressionArgumentsNewTarget:new.targetImportMeta:import.metaLeftHandSideExpression:OptionalExpressionUpdateExpression:LeftHandSideExpression++LeftHandSideExpression--++UnaryExpression--UnaryExpressionUnaryExpression:deleteUnaryExpressionvoidUnaryExpressiontypeofUnaryExpression+UnaryExpression-UnaryExpression~UnaryExpression!UnaryExpressionAwaitExpressionExponentiationExpression:UpdateExpression**ExponentiationExpressionMultiplicativeExpression:MultiplicativeExpressionMultiplicativeOperatorExponentiationExpressionAdditiveExpression:AdditiveExpression+MultiplicativeExpressionAdditiveExpression-MultiplicativeExpressionShiftExpression:ShiftExpression<<AdditiveExpressionShiftExpression>>AdditiveExpressionShiftExpression>>>AdditiveExpressionRelationalExpression:RelationalExpression<ShiftExpressionRelationalExpression>ShiftExpressionRelationalExpression<=ShiftExpressionRelationalExpression>=ShiftExpressionRelationalExpressioninstanceofShiftExpressionRelationalExpressioninShiftExpressionPrivateIdentifierinShiftExpressionEqualityExpression:EqualityExpression==RelationalExpressionEqualityExpression!=RelationalExpressionEqualityExpression===RelationalExpressionEqualityExpression!==RelationalExpressionBitwiseANDExpression:BitwiseANDExpression&EqualityExpressionBitwiseXORExpression:BitwiseXORExpression^BitwiseANDExpressionBitwiseORExpression:BitwiseORExpression|BitwiseXORExpressionLogicalANDExpression:LogicalANDExpression&&BitwiseORExpressionLogicalORExpression:LogicalORExpression||LogicalANDExpressionCoalesceExpression:CoalesceExpressionHead??BitwiseORExpressionConditionalExpression:ShortCircuitExpression?AssignmentExpression:AssignmentExpressionAssignmentExpression:YieldExpressionArrowFunctionAsyncArrowFunctionLeftHandSideExpression=AssignmentExpressionLeftHandSideExpressionAssignmentOperatorAssignmentExpressionLeftHandSideExpression&&=AssignmentExpressionLeftHandSideExpression||=AssignmentExpressionLeftHandSideExpression??=AssignmentExpressionExpression:Expression,AssignmentExpression
  1. Returninvalid.

8.5.5 Static Semantics: PropName

PropertyDefinition:IdentifierReference
  1. ReturnStringValueofIdentifierReference.
PropertyDefinition:...AssignmentExpression
  1. Returnempty.
PropertyDefinition:PropertyName:AssignmentExpression
  1. ReturnPropNameofPropertyName.
LiteralPropertyName:IdentifierName
  1. ReturnStringValueofIdentifierName.
LiteralPropertyName:StringLiteral
  1. Return theSVofStringLiteral.
LiteralPropertyName:NumericLiteral
  1. Let nbr be theNumericValueofNumericLiteral.
  2. Return ! ToString(nbr).
ComputedPropertyName:[AssignmentExpression]
  1. Returnempty.
MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}getClassElementName(){FunctionBody}setClassElementName(PropertySetParameterList){FunctionBody}
  1. ReturnPropNameofClassElementName.
GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}
  1. ReturnPropNameofClassElementName.
AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}
  1. ReturnPropNameofClassElementName.
ClassElement:;
  1. Returnempty.
AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}
  1. ReturnPropNameofClassElementName.
FieldDefinition:ClassElementNameInitializeropt
  1. ReturnPropNameofClassElementName.
ClassElementName:PrivateIdentifier
  1. Returnempty.

9 Executable Code and Execution Contexts

9.1 Environment Records

Environment Record is a specification type used to define the association ofIdentifiers to specific variables and functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated with some specific syntactic structure of ECMAScript code such as aFunctionDeclaration, aBlockStatement, or aCatchclause of aTryStatement. Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that are created by that code.

Every Environment Record has an [[OuterEnv]] field, which is eithernullor a reference to an outer Environment Record. This is used to model the logical nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may, of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for multiple inner Environment Records. For example, if aFunctionDeclarationcontains two nestedFunctionDeclarations then the Environment Records of each of the nested functions will have as their outer Environment Record the Environment Record of the current evaluation of the surrounding function.

Environment Records are purely specification mechanisms and need not correspond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such values.

9.1.1 The Environment Record Type Hierarchy

Environment Records can be thought of as existing in a simple object-oriented hierarchy whereEnvironment Recordis an abstract class with three concrete subclasses:declarative Environment Record,object Environment Record, andglobal Environment Record. Function Environment Records and module Environment Records are subclasses ofdeclarative Environment Record.

TheEnvironment Recordabstract class includes the abstract specification methods defined inTable 19. These abstract methods have distinct concrete algorithms for each of the concrete subclasses.

Table 19: Abstract Methods of Environment Records
MethodPurpose
HasBinding(N)Determine if anEnvironment Recordhas a binding for the String value N. Returntrueif it does andfalseif it does not.
CreateMutableBinding(N, D)Create a new but uninitialized mutable binding in anEnvironment Record. The String value N is the text of the bound name. If the Boolean argument D istruethe binding may be subsequently deleted.
CreateImmutableBinding(N, S)Create a new but uninitialized immutable binding in anEnvironment Record. The String value N is the text of the bound name. If S istruethen attempts to set it after it has been initialized will always throw an exception, regardless of the strict mode setting of operations that reference that binding.
InitializeBinding(N, V)Set the value of an already existing but uninitialized binding in anEnvironment Record. The String value N is the text of the bound name. V is the value for the binding and is a value of anyECMAScript language type.
SetMutableBinding(N, V, S)Set the value of an already existing mutable binding in anEnvironment Record. The String value N is the text of the bound name. V is the value for the binding and may be a value of anyECMAScript language type. S is a Boolean flag. If S istrueand the binding cannot be set throw aTypeErrorexception.
GetBindingValue(N, S)Returns the value of an already existing binding from anEnvironment Record. The String value N is the text of the bound name. S is used to identify references originating instrict mode codeor that otherwise require strict mode reference semantics. If S istrueand the binding does not exist throw aReferenceErrorexception. If the binding exists but is uninitialized aReferenceErroris thrown, regardless of the value of S.
DeleteBinding(N)Delete a binding from anEnvironment Record. The String value N is the text of the bound name. If a binding for N exists, remove the binding and returntrue. If the binding exists but cannot be removed returnfalse. If the binding does not exist returntrue.
HasThisBinding()Determine if anEnvironment Recordestablishes a this binding. Returntrueif it does andfalseif it does not.
HasSuperBinding()Determine if anEnvironment Recordestablishes a super method binding. Returntrueif it does andfalseif it does not.
WithBaseObject()If thisEnvironment Recordis associated with a with statement, return the with object. Otherwise, returnundefined.

9.1.1.1 Declarative Environment Records

Each declarative Environment Record is associated with an ECMAScript program scope containing variable, constant, let, class, module, import, and/or function declarations. A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.

The behaviour of the concrete specification methods for declarative Environment Records is defined by the following algorithms.

9.1.1.1.1 HasBinding ( N )

The HasBinding concrete method of adeclarative Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

  1. If envRec has a binding for the name that is the value of N, returntrue.
  2. Returnfalse.

9.1.1.1.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates a new mutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If D has the valuetrue, the new binding is marked as being subject to deletion. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Create a mutable binding in envRec for N and record that it is uninitialized. If D istrue, record that the newly created binding may be deleted by a subsequent DeleteBinding call.
  3. ReturnNormalCompletion(empty).

9.1.1.1.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If S has the valuetrue, the new binding is marked as a strict binding. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Create an immutable binding in envRec for N and record that it is uninitialized. If S istrue, record that the newly created binding is a strict binding.
  3. ReturnNormalCompletion(empty).

9.1.1.1.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist. It performs the following steps when called:

  1. Assert: envRec must have an uninitialized binding for N.
  2. Set the bound value for N in envRec to V.
  3. Recordthat the binding for N in envRec has been initialized.
  4. ReturnNormalCompletion(empty).

9.1.1.1.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of adeclarative Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. A binding for N normally already exists, but in rare cases it may not. If the binding is an immutable binding, aTypeErroris thrown if S istrue. It performs the following steps when called:

  1. If envRec does not have a binding for N, then
    1. If S istrue, throw aReferenceErrorexception.
    2. Perform envRec.CreateMutableBinding(N,true).
    3. Perform envRec.InitializeBinding(N, V).
    4. ReturnNormalCompletion(empty).
  2. If the binding for N in envRec is a strict binding, set S totrue.
  3. If the binding for N in envRec has not yet been initialized, throw aReferenceErrorexception.
  4. Else if the binding for N in envRec is a mutable binding, change its bound value to V.
  5. Else,
    1. Assert: This is an attempt to change the value of an immutable binding.
    2. If S istrue, throw aTypeErrorexception.
  6. ReturnNormalCompletion(empty).
Note

An example of ECMAScript code that results in a missing binding at step1is:

function f() { eval("var x; x = (delete x, 0);"); }

9.1.1.1.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of adeclarative Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. If the binding exists but is uninitialized aReferenceErroris thrown, regardless of the value of S. It performs the following steps when called:

  1. Assert: envRec has a binding for N.
  2. If the binding for N in envRec is an uninitialized binding, throw aReferenceErrorexception.
  3. Return the value currently bound to N in envRec.

9.1.1.1.7 DeleteBinding ( N )

The DeleteBinding concrete method of adeclarative Environment RecordenvRec takes argument N (a String). It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

  1. Assert: envRec has a binding for the name that is the value of N.
  2. If the binding for N in envRec cannot be deleted, returnfalse.
  3. Remove the binding for N from envRec.
  4. Returntrue.

9.1.1.1.8 HasThisBinding ( )

The HasThisBinding concrete method of adeclarative Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnfalse.
Note

A regulardeclarative Environment Record(i.e., one that is neither afunction Environment Recordnor amodule Environment Record) does not provide a this binding.

9.1.1.1.9 HasSuperBinding ( )

The HasSuperBinding concrete method of adeclarative Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnfalse.
Note

A regulardeclarative Environment Record(i.e., one that is neither afunction Environment Recordnor amodule Environment Record) does not provide a super binding.

9.1.1.1.10 WithBaseObject ( )

The WithBaseObject concrete method of adeclarative Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnundefined.

9.1.1.2 Object Environment Records

Each object Environment Record is associated with an object called its binding object. An object Environment Record binds the set of string identifier names that directly correspond to the property names of its binding object. Property keys that are not strings in the form of anIdentifierNameare not included in the set of bound identifiers. Both own and inherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because properties can be dynamically added and deleted from objects, the set of identifiers bound by an object Environment Record may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if the Writable attribute of the corresponding property has the valuefalse. Immutable bindings do not exist for object Environment Records.

Object Environment Records created for with statements (14.11) can provide their binding object as an implicitthisvalue for use in function calls. The capability is controlled by a Boolean [[IsWithEnvironment]] field.

Object Environment Records have the additional state fields listed inTable 20.

Table 20: Additional Fields of Object Environment Records
Field NameValueMeaning
[[BindingObject]]ObjectThe binding object of thisEnvironment Record.
[[IsWithEnvironment]]BooleanIndicates whether thisEnvironment Recordis created for a with statement.

The behaviour of the concrete specification methods for object Environment Records is defined by the following algorithms.

9.1.1.2.1 HasBinding ( N )

The HasBinding concrete method of anobject Environment RecordenvRec takes argument N (a String). It determines if its associated binding object has a property whose name is the value of the argument N. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let foundBinding be ? HasProperty(bindingObject, N).
  3. If foundBinding isfalse, returnfalse.
  4. If envRec.[[IsWithEnvironment]] isfalse, returntrue.
  5. Let unscopables be ? Get(bindingObject,@@unscopables).
  6. IfType(unscopables) is Object, then
    1. Let blocked be ! ToBoolean(?Get(unscopables, N)).
    2. If blocked istrue, returnfalse.
  7. Returntrue.

9.1.1.2.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates in anEnvironment Record's associated binding object a property whose name is the String value and initializes it to the valueundefined. If D has the valuetrue, the new property's [[Configurable]] attribute is set totrue; otherwise it is set tofalse. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Return ? DefinePropertyOrThrow(bindingObject, N, PropertyDescriptor { [[Value]]:undefined, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]: D }).
Note

Normally envRec will not have a binding for N but if it does, the semantics ofDefinePropertyOrThrowmay result in an existing binding being replaced or shadowed or cause anabrupt completionto be returned.

9.1.1.2.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of anobject Environment Recordis never used within this specification.

9.1.1.2.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. It performs the following steps when called:

  1. Return ? envRec.SetMutableBinding(N, V,false).
Note

In this specification, all uses of CreateMutableBinding for object Environment Records are immediately followed by a call to InitializeBinding for the same name. Hence, this specification does not explicitly track the initialization state of bindings in object Environment Records.

9.1.1.2.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of anobject Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to set the value of theEnvironment Record's associated binding object's property whose name is the value of the argument N to the value of argument V. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let stillExists be ? HasProperty(bindingObject, N).
  3. If stillExists isfalseand S istrue, throw aReferenceErrorexception.
  4. Return ? Set(bindingObject, N, V, S).

9.1.1.2.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of anobject Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its associated binding object's property whose name is the String value of the argument identifier N. The property should already exist but if it does not the result depends upon S. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Let value be ? HasProperty(bindingObject, N).
  3. If value isfalse, then
    1. If S isfalse, return the valueundefined; otherwise throw aReferenceErrorexception.
  4. Return ? Get(bindingObject, N).

9.1.1.2.7 DeleteBinding ( N )

The DeleteBinding concrete method of anobject Environment RecordenvRec takes argument N (a String). It can only delete bindings that correspond to properties of the environment object whose [[Configurable]] attribute have the valuetrue. It performs the following steps when called:

  1. Let bindingObject be envRec.[[BindingObject]].
  2. Return ? bindingObject.[[Delete]](N).

9.1.1.2.8 HasThisBinding ( )

The HasThisBinding concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnfalse.
Note

Object Environment Records do not provide a this binding.

9.1.1.2.9 HasSuperBinding ( )

The HasSuperBinding concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnfalse.
Note

Object Environment Records do not provide a super binding.

9.1.1.2.10 WithBaseObject ( )

The WithBaseObject concrete method of anobject Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. If envRec.[[IsWithEnvironment]] istrue, return envRec.[[BindingObject]].
  2. Otherwise, returnundefined.

9.1.1.3 Function Environment Records

A function Environment Record is adeclarative Environment Recordthat is used to represent the top-level scope of a function and, if the function is not anArrowFunction, provides a this binding. If a function is not anArrowFunctionfunction and references super, its function Environment Record also contains the state that is used to perform super method invocations from within the function.

Function Environment Records have the additional state fields listed inTable 21.

Table 21: Additional Fields of Function Environment Records
Field NameValueMeaning
[[ThisValue]]AnyThis is thethisvalue used for this invocation of the function.
[[ThisBindingStatus]]lexical|initialized|uninitializedIf the value islexical, this is anArrowFunctionand does not have a localthisvalue.
[[FunctionObject]]ObjectThefunction objectwhose invocation caused thisEnvironment Recordto be created.
[[NewTarget]]Object |undefinedIf thisEnvironment Recordwas created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]] newTarget parameter. Otherwise, its value isundefined.

Function Environment Records support all of thedeclarative Environment Recordmethods listed inTable 19and share the same specifications for all of those methods except for HasThisBinding and HasSuperBinding. In addition, function Environment Records support the methods listed inTable 22:

Table 22: Additional Methods of Function Environment Records
MethodPurpose
BindThisValue(V)Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()Return the value of thisEnvironment Record's this binding. Throws aReferenceErrorif the this binding has not been initialized.
GetSuperBase()Return the object that is the base for super property accesses bound in thisEnvironment Record. The valueundefinedindicates that super property accesses will produce runtime errors.

The behaviour of the additional concrete specification methods for function Environment Records is defined by the following algorithms:

9.1.1.3.1 BindThisValue ( V )

The BindThisValue concrete method of afunction Environment RecordenvRec takes argument V (anECMAScript language value). It performs the following steps when called:

  1. Assert: envRec.[[ThisBindingStatus]] is notlexical.
  2. If envRec.[[ThisBindingStatus]] isinitialized, throw aReferenceErrorexception.
  3. Set envRec.[[ThisValue]] to V.
  4. Set envRec.[[ThisBindingStatus]] toinitialized.
  5. Return V.

9.1.1.3.2 HasThisBinding ( )

The HasThisBinding concrete method of afunction Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. If envRec.[[ThisBindingStatus]] islexical, returnfalse; otherwise, returntrue.

9.1.1.3.3 HasSuperBinding ( )

The HasSuperBinding concrete method of afunction Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. If envRec.[[ThisBindingStatus]] islexical, returnfalse.
  2. If envRec.[[FunctionObject]].[[HomeObject]] has the valueundefined, returnfalse; otherwise, returntrue.

9.1.1.3.4 GetThisBinding ( )

The GetThisBinding concrete method of afunction Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Assert: envRec.[[ThisBindingStatus]] is notlexical.
  2. If envRec.[[ThisBindingStatus]] isuninitialized, throw aReferenceErrorexception.
  3. Return envRec.[[ThisValue]].

9.1.1.3.5 GetSuperBase ( )

The GetSuperBase concrete method of afunction Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Let home be envRec.[[FunctionObject]].[[HomeObject]].
  2. If home has the valueundefined, returnundefined.
  3. Assert:Type(home) is Object.
  4. Return ? home.[[GetPrototypeOf]]().

9.1.1.4 Global Environment Records

A global Environment Record is used to represent the outer most scope that is shared by all of the ECMAScriptScriptelements that are processed in a commonrealm. A global Environment Record provides the bindings for built-in globals (clause19), properties of theglobal object, and for all top-level declarations (8.1.9,8.1.11) that occur within aScript.

A global Environment Record is logically a single record but it is specified as a composite encapsulating anobject Environment Recordand adeclarative Environment Record. Theobject Environment Recordhas as its base object theglobal objectof the associatedRealm Record. Thisglobal objectis the value returned by the global Environment Record's GetThisBinding concrete method. Theobject Environment Recordcomponent of a global Environment Record contains the bindings for all built-in globals (clause19) and all bindings introduced by aFunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration,AsyncGeneratorDeclaration, orVariableStatementcontained in global code. The bindings for all other ECMAScript declarations in global code are contained in thedeclarative Environment Recordcomponent of the global Environment Record.

Properties may be created directly on aglobal object. Hence, theobject Environment Recordcomponent of a global Environment Record may contain both bindings created explicitly byFunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration,AsyncGeneratorDeclaration, orVariableDeclarationdeclarations and bindings created implicitly as properties of theglobal object. In order to identify which bindings were explicitly created using declarations, a global Environment Record maintains a list of the names bound using its CreateGlobalVarBinding and CreateGlobalFunctionBinding concrete methods.

Global Environment Records have the additional fields listed inTable 23and the additional methods listed inTable 24.

Table 23: Additional Fields of Global Environment Records
Field NameValueMeaning
[[ObjectRecord]]Object Environment RecordBinding object is theglobal object. It contains global built-in bindings as well asFunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration,AsyncGeneratorDeclaration, andVariableDeclarationbindings in global code for the associatedrealm.
[[GlobalThisValue]]ObjectThe value returned by this in global scope. Hosts may provide any ECMAScript Object value.
[[DeclarativeRecord]]Declarative Environment RecordContainsbindings for all declarations in global code for the associatedrealmcode except forFunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration,AsyncGeneratorDeclaration, andVariableDeclarationbindings.
[[VarNames]]Listof StringThe string names bound byFunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration,AsyncGeneratorDeclaration, andVariableDeclarationdeclarations in global code for the associatedrealm.
Table 24: Additional Methods of Global Environment Records
MethodPurpose
GetThisBinding()Return the value of thisEnvironment Record's this binding.
HasVarDeclaration (N)Determines if the argument identifier has a binding in thisEnvironment Recordthat was created using aVariableDeclaration,FunctionDeclaration,GeneratorDeclaration,AsyncFunctionDeclaration, orAsyncGeneratorDeclaration.
HasLexicalDeclaration (N)Determines if the argument identifier has a binding in thisEnvironment Recordthat was created using a lexical declaration such as aLexicalDeclarationor aClassDeclaration.
HasRestrictedGlobalProperty (N)Determines if the argument is the name of aglobal objectproperty that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N.
CanDeclareGlobalFunction (N)Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N.
CreateGlobalVarBinding(N, D)Used to create and initialize toundefineda global var binding in the [[ObjectRecord]] component of aglobal Environment Record. The binding will be a mutable binding. The correspondingglobal objectproperty will have attribute values appropriate for a var. The String value N is the bound name. If D istruethe binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)Create and initialize a global function binding in the [[ObjectRecord]] component of aglobal Environment Record. The binding will be a mutable binding. The correspondingglobal objectproperty will have attribute values appropriate for a function. The String value N is the bound name. V is the initialization value. If the Boolean argument D istruethe binding may be deleted. Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function declarations to receive special treatment.

The behaviour of the concrete specification methods for global Environment Records is defined by the following algorithms.

9.1.1.4.1 HasBinding ( N )

The HasBinding concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is one of the identifiers bound by the record. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, returntrue.
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.HasBinding(N).

9.1.1.4.2 CreateMutableBinding ( N, D )

The CreateMutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates a new mutable binding for the name N that is uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N must not already exist in the DeclarativeRecord. If D has the valuetrue, the new binding is marked as being subject to deletion. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, throw aTypeErrorexception.
  3. Return DclRec.CreateMutableBinding(N, D).

9.1.1.4.3 CreateImmutableBinding ( N, S )

The CreateImmutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It creates a new immutable binding for the name N that is uninitialized. A binding must not already exist in thisEnvironment Recordfor N. If S has the valuetrue, the new binding is marked as a strict binding. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, throw aTypeErrorexception.
  3. Return DclRec.CreateImmutableBinding(N, S).

9.1.1.4.4 InitializeBinding ( N, V )

The InitializeBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and V (anECMAScript language value). It is used to set the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. An uninitialized binding for N must already exist. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, then
    1. Return DclRec.InitializeBinding(N, V).
  3. Assert: If the binding exists, it must be in theobject Environment Record.
  4. Let ObjRec be envRec.[[ObjectRecord]].
  5. Return ? ObjRec.InitializeBinding(N, V).

9.1.1.4.5 SetMutableBinding ( N, V, S )

The SetMutableBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and S (a Boolean). It attempts to change the bound value of the current binding of the identifier whose name is the value of the argument N to the value of argument V. If the binding is an immutable binding, aTypeErroris thrown if S istrue. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, then
    1. Return DclRec.SetMutableBinding(N, V, S).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.SetMutableBinding(N, V, S).

9.1.1.4.6 GetBindingValue ( N, S )

The GetBindingValue concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. If the binding is an uninitialized binding throw aReferenceErrorexception. A property named N normally already exists but if it does not or is not currently writable, error handling is determined by S. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, then
    1. Return DclRec.GetBindingValue(N, S).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Return ? ObjRec.GetBindingValue(N, S).

9.1.1.4.7 DeleteBinding ( N )

The DeleteBinding concrete method of aglobal Environment RecordenvRec takes argument N (a String). It can only delete bindings that have been explicitly designated as being subject to deletion. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. If DclRec.HasBinding(N) istrue, then
    1. Return DclRec.DeleteBinding(N).
  3. Let ObjRec be envRec.[[ObjectRecord]].
  4. Let globalObject be ObjRec.[[BindingObject]].
  5. Let existingProp be ? HasOwnProperty(globalObject, N).
  6. If existingProp istrue, then
    1. Let status be ? ObjRec.DeleteBinding(N).
    2. If status istrue, then
      1. Let varNames be envRec.[[VarNames]].
      2. If N is an element of varNames, remove that element from the varNames.
    3. Return status.
  7. Returntrue.

9.1.1.4.8 HasThisBinding ( )

The HasThisBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returntrue.
Note

Global Environment Records always provide a this binding.

9.1.1.4.9 HasSuperBinding ( )

The HasSuperBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnfalse.
Note

Global Environment Records do not provide a super binding.

9.1.1.4.10 WithBaseObject ( )

The WithBaseObject concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnundefined.

9.1.1.4.11 GetThisBinding ( )

The GetThisBinding concrete method of aglobal Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Return envRec.[[GlobalThisValue]].

9.1.1.4.12 HasVarDeclaration ( N )

The HasVarDeclaration concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier has a binding in this record that was created using aVariableStatementor aFunctionDeclaration. It performs the following steps when called:

  1. Let varDeclaredNames be envRec.[[VarNames]].
  2. If varDeclaredNames contains N, returntrue.
  3. Returnfalse.

9.1.1.4.13 HasLexicalDeclaration ( N )

The HasLexicalDeclaration concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier has a binding in this record that was created using a lexical declaration such as aLexicalDeclarationor aClassDeclaration. It performs the following steps when called:

  1. Let DclRec be envRec.[[DeclarativeRecord]].
  2. Return DclRec.HasBinding(N).

9.1.1.4.14 HasRestrictedGlobalProperty ( N )

The HasRestrictedGlobalProperty concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if the argument identifier is the name of a property of theglobal objectthat must not be shadowed by a global lexical binding. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp isundefined, returnfalse.
  5. If existingProp.[[Configurable]] istrue, returnfalse.
  6. Returntrue.
Note

Properties may exist upon aglobal objectthat were directly created rather than being declared using a var or function declaration. A global lexical binding may not be created that has the same name as a non-configurable property of theglobal object. The global property"undefined"is an example of such a property.

9.1.1.4.15 CanDeclareGlobalVar ( N )

The CanDeclareGlobalVar concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same argument N. Redundant var declarations and var declarations for pre-existingglobal objectproperties are allowed. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let hasProperty be ? HasOwnProperty(globalObject, N).
  4. If hasProperty istrue, returntrue.
  5. Return ? IsExtensible(globalObject).

9.1.1.4.16 CanDeclareGlobalFunction ( N )

The CanDeclareGlobalFunction concrete method of aglobal Environment RecordenvRec takes argument N (a String). It determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the same argument N. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp isundefined, return ? IsExtensible(globalObject).
  5. If existingProp.[[Configurable]] istrue, returntrue.
  6. IfIsDataDescriptor(existingProp) istrueand existingProp has attribute values { [[Writable]]:true, [[Enumerable]]:true}, returntrue.
  7. Returnfalse.

9.1.1.4.17 CreateGlobalVarBinding ( N, D )

The CreateGlobalVarBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String) and D (a Boolean). It creates and initializes a mutable binding in the associatedobject Environment Recordand records the bound name in the associated [[VarNames]]List. If a binding already exists, it is reused and assumed to be initialized. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let hasProperty be ? HasOwnProperty(globalObject, N).
  4. Let extensible be ? IsExtensible(globalObject).
  5. If hasProperty isfalseand extensible istrue, then
    1. Perform ? ObjRec.CreateMutableBinding(N, D).
    2. Perform ? ObjRec.InitializeBinding(N,undefined).
  6. Let varDeclaredNames be envRec.[[VarNames]].
  7. If varDeclaredNames does not contain N, then
    1. Append N to varDeclaredNames.
  8. ReturnNormalCompletion(empty).

9.1.1.4.18 CreateGlobalFunctionBinding ( N, V, D )

The CreateGlobalFunctionBinding concrete method of aglobal Environment RecordenvRec takes arguments N (a String), V (anECMAScript language value), and D (a Boolean). It creates and initializes a mutable binding in the associatedobject Environment Recordand records the bound name in the associated [[VarNames]]List. If a binding already exists, it is replaced. It performs the following steps when called:

  1. Let ObjRec be envRec.[[ObjectRecord]].
  2. Let globalObject be ObjRec.[[BindingObject]].
  3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
  4. If existingProp isundefinedor existingProp.[[Configurable]] istrue, then
    1. Let desc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]: D }.
  5. Else,
    1. Let desc be the PropertyDescriptor { [[Value]]: V }.
  6. Perform ? DefinePropertyOrThrow(globalObject, N, desc).
  7. Perform ? Set(globalObject, N, V,false).
  8. Let varDeclaredNames be envRec.[[VarNames]].
  9. If varDeclaredNames does not contain N, then
    1. Append N to varDeclaredNames.
  10. ReturnNormalCompletion(empty).
Note

Global function declarations are always represented as own properties of theglobal object. If possible, an existing own property is reconfigured to have a standard set of attribute values. Step7is equivalent to what calling the InitializeBinding concrete method would do and if globalObject is a Proxy will produce the same sequence of Proxy trap calls.

9.1.1.5 Module Environment Records

A module Environment Record is adeclarative Environment Recordthat is used to represent the outer scope of an ECMAScriptModule. In additional to normal mutable and immutable bindings, module Environment Records also provide immutable import bindings which are bindings that provide indirect access to a target binding that exists in anotherEnvironment Record.

Module Environment Records support all of thedeclarative Environment Recordmethods listed inTable 19and share the same specifications for all of those methods except for GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, module Environment Records support the methods listed inTable 25:

Table 25: Additional Methods of Module Environment Records
MethodPurpose
CreateImportBinding(N, M, N2)Create an immutable indirect binding in amodule Environment Record. The String value N is the text of the bound name. M is aModule Record, and N2 is a binding that exists in M'smodule Environment Record.
GetThisBinding()Return the value of thisEnvironment Record's this binding.

The behaviour of the additional concrete specification methods for module Environment Records are defined by the following algorithms:

9.1.1.5.1 GetBindingValue ( N, S )

The GetBindingValue concrete method of amodule Environment RecordenvRec takes arguments N (a String) and S (a Boolean). It returns the value of its bound identifier whose name is the value of the argument N. However, if the binding is an indirect binding the value of the target binding is returned. If the binding exists but is uninitialized aReferenceErroris thrown. It performs the following steps when called:

  1. Assert: S istrue.
  2. Assert: envRec has a binding for N.
  3. If the binding for N is an indirect binding, then
    1. Let M and N2 be the indirection values provided when this binding for N was created.
    2. Let targetEnv be M.[[Environment]].
    3. If targetEnv isundefined, throw aReferenceErrorexception.
    4. Return ? targetEnv.GetBindingValue(N2,true).
  4. If the binding for N in envRec is an uninitialized binding, throw aReferenceErrorexception.
  5. Return the value currently bound to N in envRec.
Note

S will always betruebecause aModuleis alwaysstrict mode code.

9.1.1.5.2 DeleteBinding ( N )

The DeleteBinding concrete method of amodule Environment Recordis never used within this specification.

Note

Module Environment Records are only used within strict code and anearly errorrule prevents the delete operator, in strict code, from being applied to aReference Recordthat would resolve to amodule Environment Recordbinding. See13.5.1.1.

9.1.1.5.3 HasThisBinding ( )

The HasThisBinding concrete method of amodule Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returntrue.
Note

Module Environment Records always provide a this binding.

9.1.1.5.4 GetThisBinding ( )

The GetThisBinding concrete method of amodule Environment RecordenvRec takes no arguments. It performs the following steps when called:

  1. Returnundefined.

9.1.1.5.5 CreateImportBinding ( N, M, N2 )

The CreateImportBinding concrete method of amodule Environment RecordenvRec takes arguments N (a String), M (aModule Record), and N2 (a String). It creates a new initialized immutable indirect binding for the name N. A binding must not already exist in thisEnvironment Recordfor N. N2 is the name of a binding that exists in M'smodule Environment Record. Accesses to the value of the new binding will indirectly access the bound value of the target binding. It performs the following steps when called:

  1. Assert: envRec does not already have a binding for N.
  2. Assert: M is aModule Record.
  3. Assert: When M.[[Environment]] is instantiated it will have a direct binding for N2.
  4. Create an immutable indirect binding in envRec for N that references M and N2 as its target binding and record that the binding is initialized.
  5. ReturnNormalCompletion(empty).

9.1.2 Environment Record Operations

The followingabstract operationsare used in this specification to operate upon Environment Records:

9.1.2.1 GetIdentifierReference ( env, name, strict )

The abstract operation GetIdentifierReference takes arguments env (anEnvironment Recordornull), name (a String), and strict (a Boolean). It performs the following steps when called:

  1. If env is the valuenull, then
    1. Return theReference Record{ [[Base]]:unresolvable, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]:empty}.
  2. Let exists be ? env.HasBinding(name).
  3. If exists istrue, then
    1. Return theReference Record{ [[Base]]: env, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]:empty}.
  4. Else,
    1. Let outer be env.[[OuterEnv]].
    2. Return ? GetIdentifierReference(outer, name, strict).

9.1.2.2 NewDeclarativeEnvironment ( E )

The abstract operation NewDeclarativeEnvironment takes argument E (anEnvironment Record). It performs the following steps when called:

  1. Let env be a newdeclarative Environment Recordcontaining no bindings.
  2. Set env.[[OuterEnv]] to E.
  3. Return env.

9.1.2.3 NewObjectEnvironment ( O, W, E )

The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a Boolean), and E (anEnvironment Recordornull). It performs the following steps when called:

  1. Let env be a newobject Environment Record.
  2. Set env.[[BindingObject]] to O.
  3. Set env.[[IsWithEnvironment]] to W.
  4. Set env.[[OuterEnv]] to E.
  5. Return env.

9.1.2.4 NewFunctionEnvironment ( F, newTarget )

The abstract operation NewFunctionEnvironment takes arguments F and newTarget. It performs the following steps when called:

  1. Assert: F is an ECMAScript function.
  2. Assert:Type(newTarget) is Undefined or Object.
  3. Let env be a newfunction Environment Recordcontaining no bindings.
  4. Set env.[[FunctionObject]] to F.
  5. If F.[[ThisMode]] islexical, set env.[[ThisBindingStatus]] tolexical.
  6. Else, set env.[[ThisBindingStatus]] touninitialized.
  7. Set env.[[NewTarget]] to newTarget.
  8. Set env.[[OuterEnv]] to F.[[Environment]].
  9. Return env.

9.1.2.5 NewGlobalEnvironment ( G, thisValue )

The abstract operation NewGlobalEnvironment takes arguments G and thisValue. It performs the following steps when called:

  1. Let objRec beNewObjectEnvironment(G,false,null).
  2. Let dclRec be a newdeclarative Environment Recordcontaining no bindings.
  3. Let env be a newglobal Environment Record.
  4. Set env.[[ObjectRecord]] to objRec.
  5. Set env.[[GlobalThisValue]] to thisValue.
  6. Set env.[[DeclarativeRecord]] to dclRec.
  7. Set env.[[VarNames]] to a new emptyList.
  8. Set env.[[OuterEnv]] tonull.
  9. Return env.

9.1.2.6 NewModuleEnvironment ( E )

The abstract operation NewModuleEnvironment takes argument E (anEnvironment Record). It performs the following steps when called:

  1. Let env be a newmodule Environment Recordcontaining no bindings.
  2. Set env.[[OuterEnv]] to E.
  3. Return env.

9.2 PrivateEnvironment Records

A PrivateEnvironment Record is a specification mechanism used to track Private Names based upon the lexical nesting structure ofClassDeclarations andClassExpressions in ECMAScript code. They are similar to, but distinct from, Environment Records. EachPrivateEnvironment Recordis associated with aClassDeclarationorClassExpression. Each time such a class is evaluated, a newPrivateEnvironment Recordis created to record the Private Names declared by that class.

EachPrivateEnvironment Recordhas the fields defined inTable 26.

Table 26:PrivateEnvironment RecordFields
Field NameValue TypeMeaning
[[OuterPrivateEnvironment]]PrivateEnvironment Record|nullThePrivateEnvironment Recordof the nearest containing class.nullif the class with which thisPrivateEnvironment Recordis associated is not contained in any other class.
[[Names]]Listof Private Names.The Private Names declared by this class.

9.2.1 PrivateEnvironment Record Operations

The followingabstract operationsare used in this specification to operate upon PrivateEnvironment Records:

9.2.1.1 NewPrivateEnvironment ( outerPrivEnv )

The abstract operation NewPrivateEnvironment takes argument outerPrivEnv (aPrivateEnvironment Recordornull). It performs the following steps when called:

  1. Let names be a new emptyList.
  2. Return thePrivateEnvironment Record{ [[OuterPrivateEnvironment]]: outerPrivEnv, [[Names]]: names }.

9.2.1.2 ResolvePrivateIdentifier ( privEnv, identifier )

The abstract operation ResolvePrivateIdentifier takes arguments privEnv (aPrivateEnvironment Record) and identifier (a String). It performs the following steps when called:

  1. Let names be privEnv.[[Names]].
  2. If names contains aPrivate Namewhose [[Description]] is identifier, then
    1. Let name be thatPrivate Name.
    2. Return name.
  3. Else,
    1. Let outerPrivEnv be privEnv.[[OuterPrivateEnvironment]].
    2. Assert: outerPrivEnv is notnull.
    3. ReturnResolvePrivateIdentifier(outerPrivEnv, identifier).

9.3 Realms

Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, arealmconsists of a set of intrinsic objects, an ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global environment, and other associated state and resources.

Arealmis represented in this specification as a Realm Record with the fields specified inTable 27:

Table 27:Realm RecordFields
Field NameValueMeaning
[[Intrinsics]]Recordwhose field names are intrinsic keys and whose values are objectsThe intrinsic values used by code associated with thisrealm
[[GlobalObject]]ObjectTheglobal objectfor thisrealm
[[GlobalEnv]]global Environment RecordThe global environment for thisrealm
[[TemplateMap]]AListofRecord{ [[Site]]:Parse Node, [[Array]]: Object }.

Template objects are canonicalized separately for eachrealmusing itsRealm Record's [[TemplateMap]]. Each [[Site]] value is aParse Nodethat is aTemplateLiteral. The associated [[Array]] value is the corresponding template object that is passed to a tag function.

Note
Once aParse Nodebecomes unreachable, the corresponding [[Array]] is also unreachable, and it would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
[[HostDefined]]Any, default value isundefined.Field reserved for use by hosts that need to associate additional information with aRealm Record.

9.3.1 CreateRealm ( )

The abstract operation CreateRealm takes no arguments. It performs the following steps when called:

  1. Let realmRec be a newRealm Record.
  2. PerformCreateIntrinsics(realmRec).
  3. Set realmRec.[[GlobalObject]] toundefined.
  4. Set realmRec.[[GlobalEnv]] toundefined.
  5. Set realmRec.[[TemplateMap]] to a new emptyList.
  6. Return realmRec.

9.3.2 CreateIntrinsics ( realmRec )

The abstract operation CreateIntrinsics takes argument realmRec. It performs the following steps when called:

  1. Let intrinsics be a newRecord.
  2. Set realmRec.[[Intrinsics]] to intrinsics.
  3. Set fields of intrinsics with the values listed inTable 8. The field names are the names listed in column one of the table. The value of each field is a new object value fully and recursively populated with property values as defined by the specification of each object in clauses19through28. All object property values are newly created object values. All values that are built-in function objects are created by performingCreateBuiltinFunction(steps, length, name, slots, realmRec, prototype) where steps is the definition of that function provided by this specification, name is the initial value of the function's name property, length is the initial value of the function's length property, slots is a list of the names, if any, of the function's specified internal slots, and prototype is the specified value of the function's [[Prototype]] internal slot. The creation of the intrinsics and their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
  4. PerformAddRestrictedFunctionProperties(intrinsics.[[%Function.prototype%]], realmRec).
  5. Return intrinsics.

9.3.3 SetRealmGlobalObject ( realmRec, globalObj, thisValue )

The abstract operation SetRealmGlobalObject takes arguments realmRec, globalObj, and thisValue. It performs the following steps when called:

  1. If globalObj isundefined, then
    1. Let intrinsics be realmRec.[[Intrinsics]].
    2. Set globalObj to ! OrdinaryObjectCreate(intrinsics.[[%Object.prototype%]]).
  2. Assert:Type(globalObj) is Object.
  3. If thisValue isundefined, set thisValue to globalObj.
  4. Set realmRec.[[GlobalObject]] to globalObj.
  5. Let newGlobalEnv beNewGlobalEnvironment(globalObj, thisValue).
  6. Set realmRec.[[GlobalEnv]] to newGlobalEnv.
  7. Return realmRec.

9.3.4 SetDefaultGlobalBindings ( realmRec )

The abstract operation SetDefaultGlobalBindings takes argument realmRec. It performs the following steps when called:

  1. Let global be realmRec.[[GlobalObject]].
  2. For each property of the Global Object specified in clause19, do
    1. Let name be the String value of theproperty name.
    2. Let desc be the fully populated dataProperty Descriptorfor the property, containing the specified attributes for the property. For properties listed in19.2,19.3, or19.4the value of the [[Value]] attribute is the corresponding intrinsic object from realmRec.
    3. Perform ? DefinePropertyOrThrow(global, name, desc).
  3. Return global.

9.4 Execution Contexts

An execution context is a specification device that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is at most one execution context peragentthat is actually executing code. This is known as theagent's running execution context. All references to therunning execution contextin this specification denote therunning execution contextof thesurrounding agent.

The execution context stack is used to track execution contexts. Therunning execution contextis always the top element of this stack. A new execution context is created whenever control is transferred from the executable code associated with the currentlyrunning execution contextto executable code that is not associated with that execution context. The newly created execution context is pushed onto the stack and becomes therunning execution context.

An execution context contains whatever implementation specific state is necessary to track the execution progress of its associated code. Each execution context has at least the state components listed inTable 28.

Table 28: State Components for All Execution Contexts
ComponentPurpose
code evaluation stateAny state needed to perform, suspend, and resume evaluation of the code associated with thisexecution context.
FunctionIf thisexecution contextis evaluating the code of afunction object, then the value of this component is thatfunction object. If the context is evaluating the code of aScriptorModule, the value isnull.
RealmTheRealm Recordfrom which associated code accesses ECMAScript resources.
ScriptOrModuleTheModule RecordorScript Recordfrom which associated code originates. If there is no originating script or module, as is the case for the originalexecution contextcreated inInitializeHostDefinedRealm, the value isnull.

Evaluation of code by therunning execution contextmay be suspended at various points defined within this specification. Once therunning execution contexthas been suspended a different execution context may become therunning execution contextand commence evaluating its code. At some later time a suspended execution context may again become therunning execution contextand continue evaluating its code at the point where it had previously been suspended. Transition of therunning execution contextstatus among execution contexts usually occurs in stack-like last-in/first-out manner. However, some ECMAScript features require non-LIFO transitions of therunning execution context.

The value of theRealmcomponent of therunning execution contextis also called the current Realm Record. The value of the Function component of therunning execution contextis also called the active function object.

Execution contexts for ECMAScript code have the additional state components listed inTable 29.

Table 29: Additional State Components for ECMAScript Code Execution Contexts
ComponentPurpose
LexicalEnvironmentIdentifies theEnvironment Recordused to resolve identifier references made by code within thisexecution context.
VariableEnvironmentIdentifies theEnvironment Recordthat holds bindings created byVariableStatements within thisexecution context.
PrivateEnvironmentIdentifies thePrivateEnvironment Recordthat holds Private Names created byClassElements in the nearest containing class.nullif there is no containing class.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Environment Records.

Execution contexts representing the evaluation of generator objects have the additional state components listed inTable 30.

Table 30: Additional State Components for Generator Execution Contexts
ComponentPurpose
GeneratorThe generator object that thisexecution contextis evaluating.

In most situations only therunning execution context(the top of theexecution context stack) is directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and “VariableEnvironment” are used without qualification they are in reference to those components of therunning execution context.

An execution context is purely a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an execution context.

9.4.1 GetActiveScriptOrModule ( )

The abstract operation GetActiveScriptOrModule takes no arguments. It is used to determine the running script or module, based on therunning execution context. It performs the following steps when called:

  1. If theexecution context stackis empty, returnnull.
  2. Let ec be the topmostexecution contexton theexecution context stackwhose ScriptOrModule component is notnull.
  3. If no suchexecution contextexists, returnnull. Otherwise, return ec's ScriptOrModule.

9.4.2 ResolveBinding ( name [ , env ] )

The abstract operation ResolveBinding takes argument name (a String) and optional argument env (anEnvironment Record). It is used to determine the binding of name. env can be used to explicitly provide theEnvironment Recordthat is to be searched for the binding. It performs the following steps when called:

  1. If env is not present or if env isundefined, then
    1. Set env to therunning execution context's LexicalEnvironment.
  2. Assert: env is anEnvironment Record.
  3. If the code matching the syntactic production that is being evaluated is contained instrict mode code, let strict betrue; else let strict befalse.
  4. Return ? GetIdentifierReference(env, name, strict).
Note

The result of ResolveBinding is always aReference Recordwhose [[ReferencedName]] field is name.

9.4.3 GetThisEnvironment ( )

The abstract operation GetThisEnvironment takes no arguments. It finds theEnvironment Recordthat currently supplies the binding of thekeywordthis. It performs the following steps when called:

  1. Let env be therunning execution context's LexicalEnvironment.
  2. Repeat,
    1. Let exists be env.HasThisBinding().
    2. If exists istrue, return env.
    3. Let outer be env.[[OuterEnv]].
    4. Assert: outer is notnull.
    5. Set env to outer.
Note

The loop in step2will always terminate because the list of environments always ends with the global environment which has a this binding.

9.4.4 ResolveThisBinding ( )

The abstract operation ResolveThisBinding takes no arguments. It determines the binding of thekeywordthis using the LexicalEnvironment of therunning execution context. It performs the following steps when called:

  1. Let envRec beGetThisEnvironment().
  2. Return ? envRec.GetThisBinding().

9.4.5 GetNewTarget ( )

The abstract operation GetNewTarget takes no arguments. It determines the NewTarget value using the LexicalEnvironment of therunning execution context. It performs the following steps when called:

  1. Let envRec beGetThisEnvironment().
  2. Assert: envRec has a [[NewTarget]] field.
  3. Return envRec.[[NewTarget]].

9.4.6 GetGlobalObject ( )

The abstract operation GetGlobalObject takes no arguments. It returns theglobal objectused by the currentlyrunning execution context. It performs the following steps when called:

  1. Let currentRealm bethe current Realm Record.
  2. Return currentRealm.[[GlobalObject]].

9.5 Jobs and Host Operations to Enqueue Jobs

A Job is anAbstract Closurewith no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently in progress.

Jobs are scheduled for execution by ECMAScripthostenvironments. This specification describes thehost hookHostEnqueuePromiseJobto schedule one kind of job; hosts may define additionalabstract operationswhich schedule jobs. Such operations accept aJobAbstract Closureas the parameter and schedule it to be performed at some future time. Their implementations must conform to the following requirements:

Note 1
Hostenvironments are not required to treat Jobs uniformly with respect to scheduling. For example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features may add Jobs that are not treated at such a high priority.

At any particular time, scriptOrModule (aScript Record, aModule Record, ornull) is the active script or module if all of the following conditions are true:

At any particular time, an execution is prepared to evaluate ECMAScript code if all of the following conditions are true:

Note 2

Hostenvironments may prepare an execution to evaluate code by pushing execution contexts onto theexecution context stack. The specific steps areimplementation-defined.

The specific choice ofRealmis up to thehost environment. This initialexecution contextandRealmis only in use before any callback function is invoked. When a callback function related to aJob, like a Promise handler, is invoked, the invocation pushes its ownexecution contextandRealm.

Particular kinds of Jobs have additional conformance requirements.

9.5.1 JobCallback Records

A JobCallback Record is aRecordvalue used to store afunction objectand ahost-definedvalue. Function objects that are invoked via aJobenqueued by thehostmay have additionalhost-definedcontext. To propagate the state,JobAbstract Closures should not capture and call function objects directly. Instead, useHostMakeJobCallbackandHostCallJobCallback.

Note

The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses thehost-definedvalue to propagate the incumbent settings object for Promise callbacks.

JobCallback Records have the fields listed inTable 31.

Table 31:JobCallback RecordFields
Field NameValueMeaning
[[Callback]]Afunction objectThe function to invoke when theJobis invoked.
[[HostDefined]]Any, default value isempty.Field reserved for use by hosts.

9.5.2 HostMakeJobCallback ( callback )

Thehost-definedabstract operation HostMakeJobCallback takes argument callback (afunction object).

An implementation of HostMakeJobCallback must conform to the following requirements:

The default implementation of HostMakeJobCallback performs the following steps when called:

  1. Assert:IsCallable(callback) istrue.
  2. Return theJobCallback Record{ [[Callback]]: callback, [[HostDefined]]:empty}.

ECMAScript hosts that are not web browsers must use the default implementation of HostMakeJobCallback.

Note

This is called at the time that the callback is passed to the function that is responsible for its being eventually scheduled and run. For example, promise.then(thenAction) calls MakeJobCallback on thenAction at the time of invoking Promise.prototype.then, not at the time of scheduling the reactionJob.

9.5.3 HostCallJobCallback ( jobCallback, V, argumentsList )

Thehost-definedabstract operation HostCallJobCallback takes arguments jobCallback (aJobCallback Record), V (anECMAScript language value), and argumentsList (aListof ECMAScript language values).

An implementation of HostCallJobCallback must conform to the following requirements:

  • It must perform and return the result ofCall(jobCallback.[[Callback]], V, argumentsList).
Note

This requirement means that hosts cannot change the [[Call]] behaviour of function objects defined in this specification.

The default implementation of HostCallJobCallback performs the following steps when called:

  1. Assert:IsCallable(jobCallback.[[Callback]]) istrue.
  2. Return ? Call(jobCallback.[[Callback]], V, argumentsList).

ECMAScript hosts that are not web browsers must use the default implementation of HostCallJobCallback.

9.5.4 HostEnqueuePromiseJob ( job, realm )

Thehost-definedabstract operation HostEnqueuePromiseJob takes arguments job (aJobAbstract Closure) and realm (aRealm Recordornull). It schedules job to be performed at some future time. The Abstract Closures used with this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal priority to Promise handling operations.

An implementation of HostEnqueuePromiseJob must conform to the requirements in9.5as well as the following:

Note

The realm for Jobs returned byNewPromiseResolveThenableJobis usually the result of callingGetFunctionRealmon the thenfunction object. The realm for Jobs returned byNewPromiseReactionJobis usually the result of callingGetFunctionRealmon the handler if the handler is notundefined. If the handler isundefined, realm isnull. For both kinds of Jobs, whenGetFunctionRealmcompletes abnormally (i.e. called on a revoked Proxy), realm is the currentRealmat the time of theGetFunctionRealmcall. When the realm isnull, no user ECMAScript code will be evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses realm to check for the ability to run script and for the entry concept.

9.6 InitializeHostDefinedRealm ( )

The abstract operation InitializeHostDefinedRealm takes no arguments. It performs the following steps when called:

  1. Let realm beCreateRealm().
  2. Let newContext be a newexecution context.
  3. Set the Function of newContext tonull.
  4. Set theRealmof newContext to realm.
  5. Set the ScriptOrModule of newContext tonull.
  6. Push newContext onto theexecution context stack; newContext is now therunning execution context.
  7. If thehostrequires use of anexotic objectto serve as realm'sglobal object, let global be such an object created in ahost-definedmanner. Otherwise, let global beundefined, indicating that anordinary objectshould be created as theglobal object.
  8. If thehostrequires that the this binding in realm's global scope return an object other than theglobal object, let thisValue be such an object created in ahost-definedmanner. Otherwise, let thisValue beundefined, indicating that realm's global this binding should be theglobal object.
  9. PerformSetRealmGlobalObject(realm, global, thisValue).
  10. Let globalObj be ? SetDefaultGlobalBindings(realm).
  11. Create anyhost-definedglobal objectproperties on globalObj.
  12. ReturnNormalCompletion(empty).

9.7 Agents

An agent comprises a set of ECMAScript execution contexts, anexecution context stack, arunning execution context, an Agent Record, and an executing thread. Except for theexecuting thread, the constituents of anagentbelong exclusively to thatagent.

Anagent'sexecuting threadexecutes a job on theagent's execution contexts independently of other agents, except that anexecuting threadmay be used as theexecuting threadby multiple agents, provided none of the agents sharing the thread have anAgent Recordwhose [[CanBlock]] property istrue.

Note 1

Some web browsers share a singleexecuting threadacross multiple unrelated tabs of a browser window, for example.

While anagent'sexecuting threadexecutes jobs, theagentis the surrounding agent for the code in those jobs. The code uses thesurrounding agentto access the specification level execution objects held within theagent: therunning execution context, theexecution context stack, and theAgent Record's fields.

Table 32:Agent RecordFields
Field NameValueMeaning
[[LittleEndian]]BooleanThe default value computed for the isLittleEndian parameter when it is needed by the algorithmsGetValueFromBufferandSetValueInBuffer. The choice isimplementation-definedand should be the alternative that is most efficient for the implementation. Once the value has been observed it cannot change.
[[CanBlock]]BooleanDetermines whether theagentcan block or not.
[[Signifier]]Any globally-unique valueUniquely identifies theagentwithin itsagent cluster.
[[IsLockFree1]]Booleantrueif atomic operations on one-byte values are lock-free,falseotherwise.
[[IsLockFree2]]Booleantrueif atomic operations on two-byte values are lock-free,falseotherwise.
[[IsLockFree8]]Booleantrueif atomic operations on eight-byte values are lock-free,falseotherwise.
[[CandidateExecution]]Acandidate executionRecordSee thememory model.
[[KeptAlive]]Listof objectsInitially a new emptyList, representing the list of objects to be kept alive until the end of the currentJob

Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by anyagentin theagent clusterthey cannot change.

Note 2

The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but may also reflect implementation choices that can vary over time and between ECMAScript implementations.

There is no [[IsLockFree4]] property: 4-byte atomic operations are always lock-free.

In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to complete a lock-free atomic operation.

That an atomic access of size n is lock-free does not imply anything about the (perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be performed as a sequence of several separate memory accesses. SeeReadSharedMemoryandWriteSharedMemoryfor details.

Note 3

Anagentis a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.7.1 AgentSignifier ( )

The abstract operation AgentSignifier takes no arguments. It performs the following steps when called:

  1. Let AR be theAgent Recordof thesurrounding agent.
  2. Return AR.[[Signifier]].

9.7.2 AgentCanSuspend ( )

The abstract operation AgentCanSuspend takes no arguments. It performs the following steps when called:

  1. Let AR be theAgent Recordof thesurrounding agent.
  2. Return AR.[[CanBlock]].
Note

In some environments it may not be reasonable for a givenagentto suspend. For example, in a web browser environment, it may be reasonable to disallow suspending a document's main event handling thread, while still allowing workers' event handling threads to suspend.

9.8 Agent Clusters

An agent cluster is a maximal set of agents that can communicate by operating on shared memory.

Note 1

Programs within different agents may share memory by unspecified means. At a minimum, the backing memory for SharedArrayBuffer objects can be shared among the agents in the cluster.

There may be agents that can communicate by message passing that cannot share memory; they are never in the same agent cluster.

Everyagentbelongs to exactly one agent cluster.

Note 2

The agents in a cluster need not all be alive at some particular point in time. IfagentA creates anotheragentB, after which A terminates and B createsagentC, the three agents are in the same cluster if A could share some memory with B and B could share some memory with C.

All agents within a cluster must have the same value for the [[LittleEndian]] property in their respectiveAgentRecords.

Note 3

If different agents within an agent cluster have different values of [[LittleEndian]] it becomes hard to use shared memory for multi-byte data.

All agents within a cluster must have the same values for the [[IsLockFree1]] property in their respectiveAgentRecords; similarly for the [[IsLockFree2]] property.

All agents within a cluster must have different values for the [[Signifier]] property in their respectiveAgentRecords.

An embedding may deactivate (stop forward progress) or activate (resume forward progress) anagentwithout theagent's knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are deactivated indefinitely.

Note 4

The purpose of the preceding restriction is to avoid a situation where anagentdeadlocks or starves because anotheragenthas been deactivated. For example, if an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share memory with the dedicated worker of such an independent document, and the document and its dedicated worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into its window's history), and the shared worker then tries to acquire the lock, then the shared worker will be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to access the shared worker from other windows will starve.

The implication of the restriction is that it will not be possible to share memory between agents that don't belong to the same suspend/wake collective within the embedding.

An embedding may terminate anagentwithout any of theagent's cluster's other agents' prior knowledge or cooperation. If anagentis terminated not by programmatic action of its own or of anotheragentin the cluster but by forces external to the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at least one remaining member of the cluster will be able to detect the termination, with the termination data containing enough information to identify theagentthat was terminated.

Note 5

Examples of that type of termination are: operating systems or users terminating agents that are running in separate processes; the embedding itself terminating anagentthat is running in-process with the other agents when per-agentresource accounting indicates that theagentis runaway.

Prior to any evaluation of any ECMAScript code by anyagentin a cluster, the [[CandidateExecution]] field of theAgent Recordfor all agents in the cluster is set to the initialcandidate execution. The initialcandidate executionis anempty candidate executionwhose [[EventsRecords]] field is aListcontaining, for eachagent, anAgent Events Recordwhose [[AgentSignifier]] field is thatagent's signifier, and whose [[EventList]] and [[AgentSynchronizesWith]] fields are empty Lists.

Note 6

All agents in an agent cluster share the samecandidate executionin itsAgent Record's [[CandidateExecution]] field. Thecandidate executionis a specification mechanism used by thememory model.

Note 7

An agent cluster is a specification mechanism and need not correspond to any particular artefact of an ECMAScript implementation.

9.9 Forward Progress

For anagentto make forward progress is for it to perform an evaluation step according to this specification.

Anagentbecomes blocked when itsrunning execution contextwaits synchronously and indefinitely for an external event. Only agents whoseAgent Record's [[CanBlock]] property istruecan become blocked in this sense. An unblockedagentis one that is not blocked.

Implementations must ensure that:

  • every unblockedagentwith a dedicatedexecuting threadeventually makes forward progress
  • in a set of agents that share anexecuting thread, oneagenteventually makes forward progress
  • anagentdoes not cause anotheragentto become blocked except via explicit APIs that provide blocking.
Note

This, along with the liveness guarantee in thememory model, ensures that allSeqCstwrites eventually become observable to all agents.

9.10 Processing Model of WeakRef and FinalizationRegistry Objects

9.10.1 Objectives

This specification does not make any guarantees that any object will be garbage collected. Objects which are notlivemay be released after long periods of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour triggered by garbage collection.

The semantics ofWeakRefandFinalizationRegistryobjects is based on two operations which happen at particular points in time:

  • When WeakRef.prototype.deref is called, the referent (ifundefinedis not returned) is kept alive so that subsequent, synchronous accesses also return the object. This list is reset when synchronous work is done using theClearKeptObjectsabstract operation.
  • When an object which is registered with aFinalizationRegistrybecomes unreachable, a call of theFinalizationRegistry's cleanup callback may eventually be made, after synchronous ECMAScript execution completes. TheFinalizationRegistrycleanup is performed with theCleanupFinalizationRegistryabstract operation.

Neither of these actions (ClearKeptObjectsorCleanupFinalizationRegistry) may interrupt synchronous ECMAScript execution. Because hosts may assemble longer, synchronous ECMAScript execution runs, this specification defers the scheduling ofClearKeptObjectsandCleanupFinalizationRegistryto thehost environment.

Some ECMAScript implementations include garbage collector implementations which run in the background, including when ECMAScript is idle. Letting thehost environmentscheduleCleanupFinalizationRegistryallows it to resume ECMAScript execution in order to run finalizer work, which may free up held values, reducing overall memory usage.

9.10.2 Liveness

For some set of objects S, a hypothetical WeakRef-oblivious execution with respect to S is an execution whereby the abstract operationWeakRefDerefof aWeakRefwhose referent is an element of S always returnsundefined.

Note 1
WeakRef-obliviousness, together with liveness, capture two notions. One, that aWeakRefitself does not keep an object alive. Two, that cycles in liveness does not imply that an object is live. To be concrete, if determining obj's liveness depends on determining the liveness of anotherWeakRefreferent, obj2, obj2's liveness cannot assume obj's liveness, which would be circular reasoning.
Note 2
WeakRef-obliviousness is defined on sets of objects instead of individual objects to account for cycles. If it were defined on individual objects, then an object in a cycle will be considered live even though its Object value is only observed via WeakRefs of other objects in the cycle.
Note 3
Colloquially, we say that an individual object is live if every set of objects containing it is live.

At any point during evaluation, a set of objects S is considered live if either of the following conditions is met:

  • Any element in S is included in anyagent's [[KeptAlive]]List.
  • There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that observes the Object value of any object in S.
Note 4
The second condition above intends to capture the intuition that an object is live if its identity is observable via non-WeakRefmeans. An object's identity may be observed by observing a strict equality comparison between objects or observing the object being used as key in a Map.
Note 5

Presence of an object in a field, an internal slot, or a property does not imply that the object is live. For example if the object in question is never passed back to the program, then it cannot be observed.

This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of aFinalizationRegistryCell record.

The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is not necessarily live either.

Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative approximations such as reachability. There is expected to be significant implementation leeway.

9.10.3 Execution

At any time, if a set of objects S is notlive, an ECMAScript implementation may perform the following steps atomically:

  1. For each element obj of S, do
    1. For eachWeakRefref such that ref.[[WeakRefTarget]] is obj, do
      1. Set ref.[[WeakRefTarget]] toempty.
    2. For eachFinalizationRegistryfg such that fg.[[Cells]] contains aRecordcell such that cell.[[WeakRefTarget]] is obj, do
      1. Set cell.[[WeakRefTarget]] toempty.
      2. Optionally, perform ! HostEnqueueFinalizationRegistryCleanupJob(fg).
    3. For each WeakMap map such that map.[[WeakMapData]] contains aRecordr such that r.[[Key]] is obj, do
      1. Set r.[[Key]] toempty.
      2. Set r.[[Value]] toempty.
    4. For each WeakSet set such that set.[[WeakSetData]] contains obj, do
      1. Replace the element of set.[[WeakSetData]] whose value is obj with an element whose value isempty.
Note 1

Together with the definition of liveness, this clause prescribes legal optimizations that an implementation may apply regarding WeakRefs.

It is possible to access an object without observing its identity. Optimizations such as dead variable elimination and scalar replacement on properties of non-escaping objects whose identity is not observed are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to such objects.

On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of aWeakRef, optimizations such as rematerialization that observably empty theWeakRefare prohibited.

Because callingHostEnqueueFinalizationRegistryCleanupJobis optional, registered objects in aFinalizationRegistrydo not necessarily hold thatFinalizationRegistrylive. Implementations may omitFinalizationRegistrycallbacks for any reason, e.g., if theFinalizationRegistryitself becomes dead, or if the application is shutting down.

Note 2

Implementations are not obligated to empty WeakRefs for maximal sets of non-liveobjects.

If an implementation chooses a non-liveset S in which to empty WeakRefs, it must empty WeakRefs for all objects in S simultaneously. In other words, an implementation must not empty aWeakRefpointing to an object obj without emptying out other WeakRefs that, if not emptied, could result in an execution that observes the Object value of obj.

9.10.4 Host Hooks

9.10.4.1 HostEnqueueFinalizationRegistryCleanupJob ( finalizationRegistry )

Thehost-definedabstract operation HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (aFinalizationRegistry).

Let cleanupJob be a newJobAbstract Closurewith no parameters that captures finalizationRegistry and performs the following steps when called:

  1. Let cleanupResult beCleanupFinalizationRegistry(finalizationRegistry).
  2. If cleanupResult is anabrupt completion, perform anyhost-definedsteps for reporting the error.
  3. ReturnNormalCompletion(empty).

An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be performed at some future time, if possible. It must also conform to the requirements in9.5.

9.11 ClearKeptObjects ( )

The abstract operation ClearKeptObjects takes no arguments. ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript executions completes. It performs the following steps when called:

  1. Let agentRecord be thesurrounding agent'sAgent Record.
  2. Set agentRecord.[[KeptAlive]] to a new emptyList.

9.12 AddToKeptObjects ( object )

The abstract operation AddToKeptObjects takes argument object (an Object). It performs the following steps when called:

  1. Let agentRecord be thesurrounding agent'sAgent Record.
  2. Append object to agentRecord.[[KeptAlive]].
Note
When the abstract operation AddToKeptObjects is called with a target object reference, it adds the target to a list that will point strongly at the target untilClearKeptObjectsis called.

9.13 CleanupFinalizationRegistry ( finalizationRegistry )

The abstract operation CleanupFinalizationRegistry takes argument finalizationRegistry (aFinalizationRegistry). It performs the following steps when called:

  1. Assert: finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
  2. Let callback be finalizationRegistry.[[CleanupCallback]].
  3. While finalizationRegistry.[[Cells]] contains aRecordcell such that cell.[[WeakRefTarget]] isempty, an implementation may perform the following steps:
    1. Choose any such cell.
    2. Remove cell from finalizationRegistry.[[Cells]].
    3. Perform ? HostCallJobCallback(callback,undefined, « cell.[[HeldValue]] »).
  4. ReturnNormalCompletion(empty).

10 Ordinary and Exotic Objects Behaviours

10.1 Ordinary Object Internal Methods and Internal Slots

All ordinary objects have an internal slot called [[Prototype]]. The value of this internal slot is eithernullor an object and is used for implementing inheritance. Data properties of the [[Prototype]] object are inherited (and visible as properties of the child object) for the purposes of get access, but not for set access. Accessor properties are inherited for both get access and set access.

Everyordinary objecthas a Boolean-valued [[Extensible]] internal slot which is used to fulfill the extensibility-related internal method invariants specified in6.1.7.3. Namely, once the value of an object's [[Extensible]] internal slot has been set tofalse, it is no longer possible to add properties to the object, to modify the value of the object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] totrue.

In the following algorithm descriptions, assume O is anordinary object, P is a property key value, V is anyECMAScript language value, and Desc is aProperty Descriptorrecord.

Eachordinary objectinternal method delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal method, then the internal method is invoked on O rather than calling the similarly-named abstract operation directly. These semantics ensure that exotic objects have their overridden internal methods invoked whenordinary objectinternal methods are applied to them.

10.1.1 [[GetPrototypeOf]] ( )

The [[GetPrototypeOf]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:

  1. Return ! OrdinaryGetPrototypeOf(O).

10.1.1.1 OrdinaryGetPrototypeOf ( O )

The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object). It performs the following steps when called:

  1. Return O.[[Prototype]].

10.1.2 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of anordinary objectO takes argument V (an Object ornull). It performs the following steps when called:

  1. Return ! OrdinarySetPrototypeOf(O, V).

10.1.2.1 OrdinarySetPrototypeOf ( O, V )

The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V (anECMAScript language value). It performs the following steps when called:

  1. Assert: EitherType(V) is Object orType(V) is Null.
  2. Let current be O.[[Prototype]].
  3. IfSameValue(V, current) istrue, returntrue.
  4. Let extensible be O.[[Extensible]].
  5. If extensible isfalse, returnfalse.
  6. Let p be V.
  7. Let done befalse.
  8. Repeat, while done isfalse,
    1. If p isnull, set done totrue.
    2. Else ifSameValue(p, O) istrue, returnfalse.
    3. Else,
      1. If p.[[GetPrototypeOf]] is not theordinary objectinternal method defined in10.1.1, set done totrue.
      2. Else, set p to p.[[Prototype]].
  9. Set O.[[Prototype]] to V.
  10. Returntrue.
Note

The loop in step8guarantees that there will be no circularities in any prototype chain that only includes objects that use theordinary objectdefinitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].

10.1.3 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:

  1. Return ! OrdinaryIsExtensible(O).

10.1.3.1 OrdinaryIsExtensible ( O )

The abstract operation OrdinaryIsExtensible takes argument O (an Object). It performs the following steps when called:

  1. Return O.[[Extensible]].

10.1.4 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:

  1. Return ! OrdinaryPreventExtensions(O).

10.1.4.1 OrdinaryPreventExtensions ( O )

The abstract operation OrdinaryPreventExtensions takes argument O (an Object). It performs the following steps when called:

  1. Set O.[[Extensible]] tofalse.
  2. Returntrue.

10.1.5 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of anordinary objectO takes argument P (a property key). It performs the following steps when called:

  1. Return ! OrdinaryGetOwnProperty(O, P).

10.1.5.1 OrdinaryGetOwnProperty ( O, P )

The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. If O does not have an own property with key P, returnundefined.
  3. Let D be a newly createdProperty Descriptorwith no fields.
  4. Let X be O's own property whose key is P.
  5. If X is adata property, then
    1. Set D.[[Value]] to the value of X's [[Value]] attribute.
    2. Set D.[[Writable]] to the value of X's [[Writable]] attribute.
  6. Else,
    1. Assert: X is anaccessor property.
    2. Set D.[[Get]] to the value of X's [[Get]] attribute.
    3. Set D.[[Set]] to the value of X's [[Set]] attribute.
  7. Set D.[[Enumerable]] to the value of X's [[Enumerable]] attribute.
  8. Set D.[[Configurable]] to the value of X's [[Configurable]] attribute.
  9. Return D.

10.1.6 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of anordinary objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Return ? OrdinaryDefineOwnProperty(O, P, Desc).

10.1.6.1 OrdinaryDefineOwnProperty ( O, P, Desc )

The abstract operation OrdinaryDefineOwnProperty takes arguments O (an Object), P (a property key), and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Let current be ? O.[[GetOwnProperty]](P).
  2. Let extensible be ? IsExtensible(O).
  3. ReturnValidateAndApplyPropertyDescriptor(O, P, extensible, Desc, current).

10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible, Desc, Current )

The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean), Desc (aProperty Descriptor), and Current (aProperty Descriptor). It performs the following steps when called:

  1. ReturnValidateAndApplyPropertyDescriptor(undefined,undefined, Extensible, Desc, Current).

10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P, extensible, Desc, current )

The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object orundefined), P (a property key), extensible (a Boolean), Desc (aProperty Descriptor), and current (aProperty Descriptor).

Note

Ifundefinedis passed as O, only validation is performed and no object updates are performed.

It performs the following steps when called:

  1. Assert: If O is notundefined, thenIsPropertyKey(P) istrue.
  2. If current isundefined, then
    1. If extensible isfalse, returnfalse.
    2. Assert: extensible istrue.
    3. IfIsGenericDescriptor(Desc) istrueorIsDataDescriptor(Desc) istrue, then
      1. If O is notundefined, create an owndata propertynamed P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value.
    4. Else,
      1. Assert: ! IsAccessorDescriptor(Desc) istrue.
      2. If O is notundefined, create an ownaccessor propertynamed P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attribute values are described by Desc. If the value of an attribute field of Desc is absent, the attribute of the newly created property is set to itsdefault value.
    5. Returntrue.
  3. If every field in Desc is absent, returntrue.
  4. If current.[[Configurable]] isfalse, then
    1. If Desc.[[Configurable]] is present and its value istrue, returnfalse.
    2. If Desc.[[Enumerable]] is present and ! SameValue(Desc.[[Enumerable]], current.[[Enumerable]]) isfalse, returnfalse.
  5. If ! IsGenericDescriptor(Desc) istrue, then
    1. NOTE: No further validation is required.
  6. Else if ! SameValue(!IsDataDescriptor(current), ! IsDataDescriptor(Desc)) isfalse, then
    1. If current.[[Configurable]] isfalse, returnfalse.
    2. IfIsDataDescriptor(current) istrue, then
      1. If O is notundefined, convert the property named P of object O from adata propertyto anaccessor property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values.
    3. Else,
      1. If O is notundefined, convert the property named P of object O from anaccessor propertyto adata property. Preserve the existing values of the converted property's [[Configurable]] and [[Enumerable]] attributes and set the rest of the property's attributes to theirdefault values.
  7. Else ifIsDataDescriptor(current) andIsDataDescriptor(Desc) are bothtrue, then
    1. If current.[[Configurable]] isfalseand current.[[Writable]] isfalse, then
      1. If Desc.[[Writable]] is present and Desc.[[Writable]] istrue, returnfalse.
      2. If Desc.[[Value]] is present andSameValue(Desc.[[Value]], current.[[Value]]) isfalse, returnfalse.
      3. Returntrue.
  8. Else,
    1. Assert: ! IsAccessorDescriptor(current) and ! IsAccessorDescriptor(Desc) are bothtrue.
    2. If current.[[Configurable]] isfalse, then
      1. If Desc.[[Set]] is present andSameValue(Desc.[[Set]], current.[[Set]]) isfalse, returnfalse.
      2. If Desc.[[Get]] is present andSameValue(Desc.[[Get]], current.[[Get]]) isfalse, returnfalse.
      3. Returntrue.
  9. If O is notundefined, then
    1. For each field of Desc that is present, set the corresponding attribute of the property named P of object O to the value of the field.
  10. Returntrue.

10.1.7 [[HasProperty]] ( P )

The [[HasProperty]] internal method of anordinary objectO takes argument P (a property key). It performs the following steps when called:

  1. Return ? OrdinaryHasProperty(O, P).

10.1.7.1 OrdinaryHasProperty ( O, P )

The abstract operation OrdinaryHasProperty takes arguments O (an Object) and P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let hasOwn be ? O.[[GetOwnProperty]](P).
  3. If hasOwn is notundefined, returntrue.
  4. Let parent be ? O.[[GetPrototypeOf]]().
  5. If parent is notnull, then
    1. Return ? parent.[[HasProperty]](P).
  6. Returnfalse.

10.1.8 [[Get]] ( P, Receiver )

The [[Get]] internal method of anordinary objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Return ? OrdinaryGet(O, P, Receiver).

10.1.8.1 OrdinaryGet ( O, P, Receiver )

The abstract operation OrdinaryGet takes arguments O (an Object), P (a property key), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let desc be ? O.[[GetOwnProperty]](P).
  3. If desc isundefined, then
    1. Let parent be ? O.[[GetPrototypeOf]]().
    2. If parent isnull, returnundefined.
    3. Return ? parent.[[Get]](P, Receiver).
  4. IfIsDataDescriptor(desc) istrue, return desc.[[Value]].
  5. Assert:IsAccessorDescriptor(desc) istrue.
  6. Let getter be desc.[[Get]].
  7. If getter isundefined, returnundefined.
  8. Return ? Call(getter, Receiver).

10.1.9 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of anordinary objectO takes arguments P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Return ? OrdinarySet(O, P, V, Receiver).

10.1.9.1 OrdinarySet ( O, P, V, Receiver )

The abstract operation OrdinarySet takes arguments O (an Object), P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let ownDesc be ? O.[[GetOwnProperty]](P).
  3. ReturnOrdinarySetWithOwnDescriptor(O, P, V, Receiver, ownDesc).

10.1.9.2 OrdinarySetWithOwnDescriptor ( O, P, V, Receiver, ownDesc )

The abstract operation OrdinarySetWithOwnDescriptor takes arguments O (an Object), P (a property key), V (anECMAScript language value), Receiver (anECMAScript language value), and ownDesc (aProperty Descriptororundefined). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. If ownDesc isundefined, then
    1. Let parent be ? O.[[GetPrototypeOf]]().
    2. If parent is notnull, then
      1. Return ? parent.[[Set]](P, V, Receiver).
    3. Else,
      1. Set ownDesc to the PropertyDescriptor { [[Value]]:undefined, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.
  3. IfIsDataDescriptor(ownDesc) istrue, then
    1. If ownDesc.[[Writable]] isfalse, returnfalse.
    2. IfType(Receiver) is not Object, returnfalse.
    3. Let existingDescriptor be ? Receiver.[[GetOwnProperty]](P).
    4. If existingDescriptor is notundefined, then
      1. IfIsAccessorDescriptor(existingDescriptor) istrue, returnfalse.
      2. If existingDescriptor.[[Writable]] isfalse, returnfalse.
      3. Let valueDesc be the PropertyDescriptor { [[Value]]: V }.
      4. Return ? Receiver.[[DefineOwnProperty]](P, valueDesc).
    5. Else,
      1. Assert: Receiver does not currently have a property P.
      2. Return ? CreateDataProperty(Receiver, P, V).
  4. Assert:IsAccessorDescriptor(ownDesc) istrue.
  5. Let setter be ownDesc.[[Set]].
  6. If setter isundefined, returnfalse.
  7. Perform ? Call(setter, Receiver, « V »).
  8. Returntrue.

10.1.10 [[Delete]] ( P )

The [[Delete]] internal method of anordinary objectO takes argument P (a property key). It performs the following steps when called:

  1. Return ? OrdinaryDelete(O, P).

10.1.10.1 OrdinaryDelete ( O, P )

The abstract operation OrdinaryDelete takes arguments O (an Object) and P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let desc be ? O.[[GetOwnProperty]](P).
  3. If desc isundefined, returntrue.
  4. If desc.[[Configurable]] istrue, then
    1. Remove the own property with name P from O.
    2. Returntrue.
  5. Returnfalse.

10.1.11 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of anordinary objectO takes no arguments. It performs the following steps when called:

  1. Return ! OrdinaryOwnPropertyKeys(O).

10.1.11.1 OrdinaryOwnPropertyKeys ( O )

The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object). It performs the following steps when called:

  1. Let keys be a new emptyList.
  2. For each own property key P of O such that P is anarray index, in ascending numeric index order, do
    1. Add P as the last element of keys.
  3. For each own property key P of O such thatType(P) is String and P is not anarray index, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  4. For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  5. Return keys.

10.1.12 OrdinaryObjectCreate ( proto [ , additionalInternalSlotsList ] )

The abstract operation OrdinaryObjectCreate takes argument proto (an Object ornull) and optional argument additionalInternalSlotsList (aListof names of internal slots). It is used to specify the runtime creation of new ordinary objects. additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new emptyListis used. It performs the following steps when called:

  1. Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
  2. If additionalInternalSlotsList is present, append each of its elements to internalSlotsList.
  3. Let O be ! MakeBasicObject(internalSlotsList).
  4. Set O.[[Prototype]] to proto.
  5. Return O.
Note

Although OrdinaryObjectCreate does little more than callMakeBasicObject, its use communicates the intention to create anordinary object, and not an exotic one. Thus, within this specification, it is not called by any algorithm that subsequently modifies the internal methods of the object in ways that would make the result non-ordinary. Operations that create exotic objects invokeMakeBasicObjectdirectly.

10.1.13 OrdinaryCreateFromConstructor ( constructor, intrinsicDefaultProto [ , internalSlotsList ] )

The abstract operation OrdinaryCreateFromConstructor takes arguments constructor and intrinsicDefaultProto and optional argument internalSlotsList (aListof names of internal slots). It creates anordinary objectwhose [[Prototype]] value is retrieved from aconstructor's"prototype"property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. If internalSlotsList is not provided, a new emptyListis used. It performs the following steps when called:

  1. Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. Let proto be ? GetPrototypeFromConstructor(constructor, intrinsicDefaultProto).
  3. Return ! OrdinaryObjectCreate(proto, internalSlotsList).

10.1.14 GetPrototypeFromConstructor ( constructor, intrinsicDefaultProto )

The abstract operation GetPrototypeFromConstructor takes arguments constructor and intrinsicDefaultProto. It determines the [[Prototype]] value that should be used to create an object corresponding to a specificconstructor. The value is retrieved from theconstructor's"prototype"property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is used for [[Prototype]]. It performs the following steps when called:

  1. Assert: intrinsicDefaultProto is a String value that is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
  2. Assert:IsCallable(constructor) istrue.
  3. Let proto be ? Get(constructor,"prototype").
  4. IfType(proto) is not Object, then
    1. Let realm be ? GetFunctionRealm(constructor).
    2. Set proto to realm's intrinsic object named intrinsicDefaultProto.
  5. Return proto.
Note

If constructor does not supply a [[Prototype]] value, the default value that is used is obtained from therealmof the constructor function rather than from therunning execution context.

10.1.15 RequireInternalSlot ( O, internalSlot )

The abstract operation RequireInternalSlot takes arguments O and internalSlot. It throws an exception unless O is an Object and has the given internal slot. It performs the following steps when called:

  1. IfType(O) is not Object, throw aTypeErrorexception.
  2. If O does not have an internalSlot internal slot, throw aTypeErrorexception.

10.2 ECMAScript Function Objects

ECMAScript function objects encapsulate parameterized ECMAScript code closed over a lexical environment and support the dynamic evaluation of that code. An ECMAScriptfunction objectis anordinary objectand has the same internal slots and the same internal methods as other ordinary objects. The code of an ECMAScriptfunction objectmay be eitherstrict mode code(11.2.2) ornon-strict code. An ECMAScriptfunction objectwhose code isstrict mode codeis called a strict function. One whose code is notstrict mode codeis called a non-strict function.

In addition to [[Extensible]] and [[Prototype]], ECMAScript function objects also have the internal slots listed inTable 33.

Table 33: Internal Slots of ECMAScript Function Objects
Internal SlotTypeDescription
[[Environment]]Environment RecordTheEnvironment Recordthat the function was closed over. Used as the outer environment when evaluating the code of the function.
[[PrivateEnvironment]]PrivateEnvironment Record|nullThePrivateEnvironment Recordfor Private Names that the function was closed over.nullif this function is not syntactically contained within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of the function.
[[FormalParameters]]Parse NodeThe root parse node of the source text that defines the function's formal parameter list.
[[ECMAScriptCode]]Parse NodeThe root parse node of the source text that defines the function's body.
[[ConstructorKind]]base|derivedWhether or not the function is a derived classconstructor.
[[Realm]]Realm RecordTherealmin which the function was created and which provides any intrinsic objects that are accessed when evaluating the function.
[[ScriptOrModule]]Script RecordorModule RecordThe script or module in which the function was created.
[[ThisMode]]lexical|strict|globalDefines how this references are interpreted within the formal parameters and code body of the function.lexicalmeans that this refers to thethisvalue of a lexically enclosing function.strictmeans that thethisvalue is used exactly as provided by an invocation of the function.globalmeans that athisvalue ofundefinedornullis interpreted as a reference to theglobal object, and any otherthisvalue is first passed toToObject.
[[Strict]]Booleantrueif this is astrict function,falseif this is anon-strict function.
[[HomeObject]]ObjectIf the function uses super, this is the object whose [[GetPrototypeOf]] provides the object where super property lookups begin.
[[SourceText]]sequence of Unicode code pointsThesource textthat defines the function.
[[Fields]]Listof ClassFieldDefinition RecordsIf the function is a class, this is a list of Records representing the non-static fields and corresponding initializers of the class.
[[PrivateMethods]]Listof PrivateElementsIf the function is a class, this is a list representing the non-static private methods and accessors of the class.
[[ClassFieldInitializerName]]String | Symbol |Private Name|emptyIf the function is created as the initializer of a class field, the name to use forNamedEvaluationof the field;emptyotherwise.
[[IsClassConstructor]]BooleanIndicates whether the function is a classconstructor. (Iftrue, invoking the function's [[Call]] will immediately throw aTypeErrorexception.)

All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.

10.2.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of an ECMAScriptfunction objectF takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:

  1. Assert: F is an ECMAScriptfunction object.
  2. Let callerContext be therunning execution context.
  3. Let calleeContext bePrepareForOrdinaryCall(F,undefined).
  4. Assert: calleeContext is now therunning execution context.
  5. If F.[[IsClassConstructor]] istrue, then
    1. Let error be a newly createdTypeErrorobject.
    2. NOTE: error is created in calleeContext with F's associatedRealm Record.
    3. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
    4. ReturnThrowCompletion(error).
  6. PerformOrdinaryCallBindThis(F, calleeContext, thisArgument).
  7. Let result beOrdinaryCallEvaluateBody(F, argumentsList).
  8. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
  9. If result.[[Type]] isreturn, returnNormalCompletion(result.[[Value]]).
  10. ReturnIfAbrupt(result).
  11. ReturnNormalCompletion(undefined).
Note

When calleeContext is removed from theexecution context stackin step8it must not be destroyed if it is suspended and retained for later resumption by an accessible generator object.

10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )

The abstract operation PrepareForOrdinaryCall takes arguments F (afunction object) and newTarget (anECMAScript language value). It performs the following steps when called:

  1. Assert:Type(newTarget) is Undefined or Object.
  2. Let callerContext be therunning execution context.
  3. Let calleeContext be a new ECMAScript codeexecution context.
  4. Set the Function of calleeContext to F.
  5. Let calleeRealm be F.[[Realm]].
  6. Set theRealmof calleeContext to calleeRealm.
  7. Set the ScriptOrModule of calleeContext to F.[[ScriptOrModule]].
  8. Let localEnv beNewFunctionEnvironment(F, newTarget).
  9. Set the LexicalEnvironment of calleeContext to localEnv.
  10. Set the VariableEnvironment of calleeContext to localEnv.
  11. Set the PrivateEnvironment of calleeContext to F.[[PrivateEnvironment]].
  12. If callerContext is not already suspended, suspend callerContext.
  13. Push calleeContext onto theexecution context stack; calleeContext is now therunning execution context.
  14. NOTE: Any exception objects produced after this point are associated with calleeRealm.
  15. Return calleeContext.

10.2.1.2 OrdinaryCallBindThis ( F, calleeContext, thisArgument )

The abstract operation OrdinaryCallBindThis takes arguments F (afunction object), calleeContext (anexecution context), and thisArgument (anECMAScript language value). It performs the following steps when called:

  1. Let thisMode be F.[[ThisMode]].
  2. If thisMode islexical, returnNormalCompletion(undefined).
  3. Let calleeRealm be F.[[Realm]].
  4. Let localEnv be the LexicalEnvironment of calleeContext.
  5. If thisMode isstrict, let thisValue be thisArgument.
  6. Else,
    1. If thisArgument isundefinedornull, then
      1. Let globalEnv be calleeRealm.[[GlobalEnv]].
      2. Assert: globalEnv is aglobal Environment Record.
      3. Let thisValue be globalEnv.[[GlobalThisValue]].
    2. Else,
      1. Let thisValue be ! ToObject(thisArgument).
      2. NOTE:ToObjectproduces wrapper objects using calleeRealm.
  7. Assert: localEnv is afunction Environment Record.
  8. Assert: The next step never returns anabrupt completionbecause localEnv.[[ThisBindingStatus]] is notinitialized.
  9. Return localEnv.BindThisValue(thisValue).

10.2.1.3 Runtime Semantics: EvaluateBody

With parameters functionObject and argumentsList (aList).

FunctionBody:FunctionStatementList
  1. Return ?EvaluateFunctionBodyofFunctionBodywith arguments functionObject and argumentsList.
ConciseBody:ExpressionBody
  1. Return ?EvaluateConciseBodyofConciseBodywith arguments functionObject and argumentsList.
GeneratorBody:FunctionBody
  1. Return ?EvaluateGeneratorBodyofGeneratorBodywith arguments functionObject and argumentsList.
AsyncGeneratorBody:FunctionBody
  1. Return ?EvaluateAsyncGeneratorBodyofAsyncGeneratorBodywith arguments functionObject and argumentsList.
AsyncFunctionBody:FunctionBody
  1. Return ?EvaluateAsyncFunctionBodyofAsyncFunctionBodywith arguments functionObject and argumentsList.
AsyncConciseBody:ExpressionBody
  1. Return ?EvaluateAsyncConciseBodyofAsyncConciseBodywith arguments functionObject and argumentsList.
Initializer:=AssignmentExpression
  1. Assert: argumentsList is empty.
  2. Assert: functionObject.[[ClassFieldInitializerName]] is notempty.
  3. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrue, then
    1. Let value beNamedEvaluationofInitializerwith argument functionObject.[[ClassFieldInitializerName]].
  4. Else,
    1. Let rhs be the result of evaluatingAssignmentExpression.
    2. Let value be ? GetValue(rhs).
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: value, [[Target]]:empty}.
Note

Even though field initializers constitute a function boundary, callingFunctionDeclarationInstantiationdoes not have any observable effect and so is omitted.

10.2.1.4 OrdinaryCallEvaluateBody ( F, argumentsList )

The abstract operation OrdinaryCallEvaluateBody takes arguments F (afunction object) and argumentsList (aList). It performs the following steps when called:

  1. Return the result ofEvaluateBodyof the parsed code that is F.[[ECMAScriptCode]] passing F and argumentsList as the arguments.

10.2.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of an ECMAScriptfunction objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:

  1. Assert: F is an ECMAScriptfunction object.
  2. Assert:Type(newTarget) is Object.
  3. Let callerContext be therunning execution context.
  4. Let kind be F.[[ConstructorKind]].
  5. If kind isbase, then
    1. Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget,"%Object.prototype%").
  6. Let calleeContext bePrepareForOrdinaryCall(F, newTarget).
  7. Assert: calleeContext is now therunning execution context.
  8. If kind isbase, then
    1. PerformOrdinaryCallBindThis(F, calleeContext, thisArgument).
    2. Let initializeResult beInitializeInstanceElements(thisArgument, F).
    3. If initializeResult is anabrupt completion, then
      1. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
      2. ReturnCompletion(initializeResult).
  9. Let constructorEnv be the LexicalEnvironment of calleeContext.
  10. Let result beOrdinaryCallEvaluateBody(F, argumentsList).
  11. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
  12. If result.[[Type]] isreturn, then
    1. IfType(result.[[Value]]) is Object, returnNormalCompletion(result.[[Value]]).
    2. If kind isbase, returnNormalCompletion(thisArgument).
    3. If result.[[Value]] is notundefined, throw aTypeErrorexception.
  13. Else,ReturnIfAbrupt(result).
  14. Return ? constructorEnv.GetThisBinding().

10.2.3 OrdinaryFunctionCreate ( functionPrototype, sourceText, ParameterList, Body, thisMode, Scope, PrivateScope )

The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object), sourceText (a sequence of Unicode code points), ParameterList (aParse Node), Body (aParse Node), thisMode (eitherlexical-thisornon-lexical-this), Scope (anEnvironment Record), and PrivateScope (aPrivateEnvironment Recordornull). sourceText is the source text of the syntactic definition of the function to be created. It performs the following steps when called:

  1. Assert:Type(functionPrototype) is Object.
  2. Let internalSlotsList be the internal slots listed inTable 33.
  3. Let F be ! OrdinaryObjectCreate(functionPrototype, internalSlotsList).
  4. Set F.[[Call]] to the definition specified in10.2.1.
  5. Set F.[[SourceText]] to sourceText.
  6. Set F.[[FormalParameters]] to ParameterList.
  7. Set F.[[ECMAScriptCode]] to Body.
  8. If the source text matching Body isstrict mode code, let Strict betrue; else let Strict befalse.
  9. Set F.[[Strict]] to Strict.
  10. If thisMode islexical-this, set F.[[ThisMode]] tolexical.
  11. Else if Strict istrue, set F.[[ThisMode]] tostrict.
  12. Else, set F.[[ThisMode]] toglobal.
  13. Set F.[[IsClassConstructor]] tofalse.
  14. Set F.[[Environment]] to Scope.
  15. Set F.[[PrivateEnvironment]] to PrivateScope.
  16. Set F.[[ScriptOrModule]] toGetActiveScriptOrModule().
  17. Set F.[[Realm]] tothe current Realm Record.
  18. Set F.[[HomeObject]] toundefined.
  19. Set F.[[Fields]] to a new emptyList.
  20. Set F.[[PrivateMethods]] to a new emptyList.
  21. Set F.[[ClassFieldInitializerName]] toempty.
  22. Let len be theExpectedArgumentCountof ParameterList.
  23. Perform ! SetFunctionLength(F, len).
  24. Return F.

10.2.4 AddRestrictedFunctionProperties ( F, realm )

The abstract operation AddRestrictedFunctionProperties takes arguments F (afunction object) and realm (aRealm Record). It performs the following steps when called:

  1. Assert: realm.[[Intrinsics]].[[%ThrowTypeError%]] exists and has been initialized.
  2. Let thrower be realm.[[Intrinsics]].[[%ThrowTypeError%]].
  3. Perform ! DefinePropertyOrThrow(F,"caller", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]:false, [[Configurable]]:true}).
  4. Return ! DefinePropertyOrThrow(F,"arguments", PropertyDescriptor { [[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]:false, [[Configurable]]:true}).

10.2.4.1 %ThrowTypeError% ( )

The %ThrowTypeError% intrinsic is an anonymous built-infunction objectthat is defined once for eachrealm. When %ThrowTypeError% is called it performs the following steps:

  1. Throw aTypeErrorexception.

The value of the [[Extensible]] internal slot of a %ThrowTypeError% function isfalse.

The"length"property of a %ThrowTypeError% function has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

The"name"property of a %ThrowTypeError% function has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

10.2.5 MakeConstructor ( F [ , writablePrototype [ , prototype ] ] )

The abstract operation MakeConstructor takes argument F (afunction object) and optional arguments writablePrototype (a Boolean) and prototype (an Object). It converts F into aconstructor. It performs the following steps when called:

  1. Assert: F is an ECMAScriptfunction objector a built-infunction object.
  2. If F is an ECMAScriptfunction object, then
    1. Assert:IsConstructor(F) isfalse.
    2. Assert: F is an extensible object that does not have a"prototype"own property.
    3. Set F.[[Construct]] to the definition specified in10.2.2.
  3. Set F.[[ConstructorKind]] tobase.
  4. If writablePrototype is not present, set writablePrototype totrue.
  5. If prototype is not present, then
    1. Set prototype to ! OrdinaryObjectCreate(%Object.prototype%).
    2. Perform ! DefinePropertyOrThrow(prototype,"constructor", PropertyDescriptor { [[Value]]: F, [[Writable]]: writablePrototype, [[Enumerable]]:false, [[Configurable]]:true}).
  6. Perform ! DefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]: writablePrototype, [[Enumerable]]:false, [[Configurable]]:false}).
  7. ReturnNormalCompletion(undefined).

10.2.6 MakeClassConstructor ( F )

The abstract operation MakeClassConstructor takes argument F. It performs the following steps when called:

  1. Assert: F is an ECMAScriptfunction object.
  2. Assert: F.[[IsClassConstructor]] isfalse.
  3. Set F.[[IsClassConstructor]] totrue.
  4. ReturnNormalCompletion(undefined).

10.2.7 MakeMethod ( F, homeObject )

The abstract operation MakeMethod takes arguments F and homeObject. It configures F as a method. It performs the following steps when called:

  1. Assert: F is an ECMAScriptfunction object.
  2. Assert:Type(homeObject) is Object.
  3. Set F.[[HomeObject]] to homeObject.
  4. ReturnNormalCompletion(undefined).

10.2.8 DefineMethodProperty ( key, homeObject, closure, enumerable )

The abstract operation DefineMethodProperty takes arguments key (a property key orPrivate Name), homeObject (an Object), closure (afunction object), and enumerable (a Boolean). It performs the following steps when called:

  1. Perform ! SetFunctionName(closure, key).
  2. If key is aPrivate Name, then
    1. ReturnPrivateElement{ [[Key]]: key, [[Kind]]:method, [[Value]]: closure }.
  3. Else,
    1. Let desc be the PropertyDescriptor { [[Value]]: closure, [[Writable]]:true, [[Enumerable]]: enumerable, [[Configurable]]:true}.
    2. Perform ? DefinePropertyOrThrow(homeObject, key, desc).
    3. Returnempty.

10.2.9 SetFunctionName ( F, name [ , prefix ] )

The abstract operation SetFunctionName takes arguments F (afunction object) and name (a property key orPrivate Name) and optional argument prefix (a String). It adds a"name"property to F. It performs the following steps when called:

  1. Assert: F is an extensible object that does not have a"name"own property.
  2. IfType(name) is Symbol, then
    1. Let description be name's [[Description]] value.
    2. If description isundefined, set name to the empty String.
    3. Else, set name to thestring-concatenationof"[", description, and"]".
  3. Else if name is aPrivate Name, then
    1. Set name to name.[[Description]].
  4. If F has an [[InitialName]] internal slot, then
    1. Set F.[[InitialName]] to name.
  5. If prefix is present, then
    1. Set name to thestring-concatenationof prefix, the code unit 0x0020 (SPACE), and name.
    2. If F has an [[InitialName]] internal slot, then
      1. Optionally, set F.[[InitialName]] to name.
  6. Return ! DefinePropertyOrThrow(F,"name", PropertyDescriptor { [[Value]]: name, [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}).

10.2.10 SetFunctionLength ( F, length )

The abstract operation SetFunctionLength takes arguments F (afunction object) and length (a non-negativeintegeror +∞). It adds a"length"property to F. It performs the following steps when called:

  1. Assert: F is an extensible object that does not have a"length"own property.
  2. Return ! DefinePropertyOrThrow(F,"length", PropertyDescriptor { [[Value]]:𝔽(length), [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}).

10.2.11 FunctionDeclarationInstantiation ( func, argumentsList )

The abstract operation FunctionDeclarationInstantiation takes arguments func (afunction object) and argumentsList. func is thefunction objectfor which theexecution contextis being established.

Note 1

When anexecution contextis established for evaluating an ECMAScript function a newfunction Environment Recordis created and bindings for each formal parameter are instantiated in thatEnvironment Record. Each declaration in the function body is also instantiated. If the function's formal parameters do not include any default value initializers then the body declarations are instantiated in the sameEnvironment Recordas the parameters. If default value parameter initializers exist, a secondEnvironment Recordis created for the body declarations. Formal parameters and functions are initialized as part of FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function body.

It performs the following steps when called:

  1. Let calleeContext be therunning execution context.
  2. Let code be func.[[ECMAScriptCode]].
  3. Let strict be func.[[Strict]].
  4. Let formals be func.[[FormalParameters]].
  5. Let parameterNames be theBoundNamesof formals.
  6. If parameterNames has any duplicate entries, let hasDuplicates betrue. Otherwise, let hasDuplicates befalse.
  7. Let simpleParameterList beIsSimpleParameterListof formals.
  8. Let hasParameterExpressions beContainsExpressionof formals.
  9. Let varNames be theVarDeclaredNamesof code.
  10. Let varDeclarations be theVarScopedDeclarationsof code.
  11. Let lexicalNames be theLexicallyDeclaredNamesof code.
  12. Let functionNames be a new emptyList.
  13. Let functionsToInitialize be a new emptyList.
  14. For each element d of varDeclarations, in reverseListorder, do
    1. If d is neither aVariableDeclarationnor aForBindingnor aBindingIdentifier, then
      1. Assert: d is either aFunctionDeclaration, aGeneratorDeclaration, anAsyncFunctionDeclaration, or anAsyncGeneratorDeclaration.
      2. Let fn be the sole element of theBoundNamesof d.
      3. If fn is not an element of functionNames, then
        1. Insert fn as the first element of functionNames.
        2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
        3. Insert d as the first element of functionsToInitialize.
  15. Let argumentsObjectNeeded betrue.
  16. If func.[[ThisMode]] islexical, then
    1. NOTE: Arrow functions never have an arguments objects.
    2. Set argumentsObjectNeeded tofalse.
  17. Else if"arguments"is an element of parameterNames, then
    1. Set argumentsObjectNeeded tofalse.
  18. Else if hasParameterExpressions isfalse, then
    1. If"arguments"is an element of functionNames or if"arguments"is an element of lexicalNames, then
      1. Set argumentsObjectNeeded tofalse.
  19. If strict istrueor if hasParameterExpressions isfalse, then
    1. NOTE: Only a singleEnvironment Recordis needed for the parameters and top-level vars.
    2. Let env be the LexicalEnvironment of calleeContext.
  20. Else,
    1. NOTE: A separateEnvironment Recordis needed to ensure that bindings created bydirect evalcalls in the formal parameter list are outside the environment where parameters are declared.
    2. Let calleeEnv be the LexicalEnvironment of calleeContext.
    3. Let env beNewDeclarativeEnvironment(calleeEnv).
    4. Assert: The VariableEnvironment of calleeContext is calleeEnv.
    5. Set the LexicalEnvironment of calleeContext to env.
  21. For each String paramName of parameterNames, do
    1. Let alreadyDeclared be env.HasBinding(paramName).
    2. NOTE: Early errors ensure that duplicate parameter names can only occur in non-strict functions that do not have parameter default values or rest parameters.
    3. If alreadyDeclared isfalse, then
      1. Perform ! env.CreateMutableBinding(paramName,false).
      2. If hasDuplicates istrue, then
        1. Perform ! env.InitializeBinding(paramName,undefined).
  22. If argumentsObjectNeeded istrue, then
    1. If strict istrueor if simpleParameterList isfalse, then
      1. Let ao beCreateUnmappedArgumentsObject(argumentsList).
    2. Else,
      1. NOTE: A mapped argument object is only provided for non-strict functions that don't have a rest parameter, any parameter default value initializers, or any destructured parameters.
      2. Let ao beCreateMappedArgumentsObject(func, formals, argumentsList, env).
    3. If strict istrue, then
      1. Perform ! env.CreateImmutableBinding("arguments",false).
    4. Else,
      1. Perform ! env.CreateMutableBinding("arguments",false).
    5. Call env.InitializeBinding("arguments", ao).
    6. Let parameterBindings be thelist-concatenationof parameterNames and «"arguments"».
  23. Else,
    1. Let parameterBindings be parameterNames.
  24. Let iteratorRecord beCreateListIteratorRecord(argumentsList).
  25. If hasDuplicates istrue, then
    1. Perform ?IteratorBindingInitializationfor formals with iteratorRecord andundefinedas arguments.
  26. Else,
    1. Perform ?IteratorBindingInitializationfor formals with iteratorRecord and env as arguments.
  27. If hasParameterExpressions isfalse, then
    1. NOTE: Only a singleEnvironment Recordis needed for the parameters and top-level vars.
    2. Let instantiatedVarNames be a copy of theListparameterBindings.
    3. For each element n of varNames, do
      1. If n is not an element of instantiatedVarNames, then
        1. Append n to instantiatedVarNames.
        2. Perform ! env.CreateMutableBinding(n,false).
        3. Call env.InitializeBinding(n,undefined).
    4. Let varEnv be env.
  28. Else,
    1. NOTE: A separateEnvironment Recordis needed to ensure that closures created by expressions in the formal parameter list do not have visibility of declarations in the function body.
    2. Let varEnv beNewDeclarativeEnvironment(env).
    3. Set the VariableEnvironment of calleeContext to varEnv.
    4. Let instantiatedVarNames be a new emptyList.
    5. For each element n of varNames, do
      1. If n is not an element of instantiatedVarNames, then
        1. Append n to instantiatedVarNames.
        2. Perform ! varEnv.CreateMutableBinding(n,false).
        3. If n is not an element of parameterBindings or if n is an element of functionNames, let initialValue beundefined.
        4. Else,
          1. Let initialValue be ! env.GetBindingValue(n,false).
        5. Call varEnv.InitializeBinding(n, initialValue).
        6. NOTE: A var with the same name as a formal parameter initially has the same value as the corresponding initialized parameter.
  29. NOTE: AnnexB.3.2.1adds additional steps at this point.
  30. If strict isfalse, then
    1. Let lexEnv beNewDeclarativeEnvironment(varEnv).
    2. NOTE: Non-strict functions use a separateEnvironment Recordfor top-level lexical declarations so that adirect evalcan determine whether any var scoped declarations introduced by the eval code conflict with pre-existing top-level lexically scoped declarations. This is not needed for strict functions because a strictdirect evalalways places all declarations into a newEnvironment Record.
  31. Else, let lexEnv be varEnv.
  32. Set the LexicalEnvironment of calleeContext to lexEnv.
  33. Let lexDeclarations be theLexicallyScopedDeclarationsof code.
  34. For each element d of lexDeclarations, do
    1. NOTE: A lexically declared name cannot be the same as a function/generator declaration, formal parameter, or a var name. Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of theBoundNamesof d, do
      1. IfIsConstantDeclarationof d istrue, then
        1. Perform ! lexEnv.CreateImmutableBinding(dn,true).
      2. Else,
        1. Perform ! lexEnv.CreateMutableBinding(dn,false).
  35. Let privateEnv be the PrivateEnvironment of calleeContext.
  36. For eachParse Nodef of functionsToInitialize, do
    1. Let fn be the sole element of theBoundNamesof f.
    2. Let fo beInstantiateFunctionObjectof f with arguments lexEnv and privateEnv.
    3. Perform ! varEnv.SetMutableBinding(fn, fo,false).
  37. ReturnNormalCompletion(empty).
Note 2

B.3.2provides an extension to the above algorithm that is necessary for backwards compatibility with web browser implementations of ECMAScript that predate ECMAScript 2015.

10.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (10.2) whose behaviour is provided using ECMAScript code or as implementation provided function exotic objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.

If a built-infunction objectis implemented as an ECMAScriptfunction object, it must have all the internal slots described in10.2([[Prototype]], [[Extensible]], and the slots listed inTable 33), plus [[InitialName]]. The value of the [[InitialName]] internal slot is a String value that is the initial name of the function. It is used by20.2.3.5.

If a built-infunction objectis implemented as anexotic object, it must have theordinary objectbehaviour specified in10.1. All such function exotic objects have [[Prototype]], [[Extensible]], [[Realm]], and [[InitialName]] internal slots, with the same meanings as above.

Unless otherwise specified every built-infunction objecthas the%Function.prototype%object as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides thethisvalue, the [[Call]] argumentsList provides the named parameters, and the NewTarget value isundefined. When invoked with [[Construct]], thethisvalue is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScriptfunction objectthen this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-inconstructorhas any [[Call]] behaviour other than throwing aTypeErrorexception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[IsClassConstructor]] internal slot to have the valuetrue.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-inconstructoris called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-inconstructor's named parameters.

Built-in functions that are not constructors do not have a"prototype"property unless otherwise specified in the description of a particular function.

If a built-infunction objectis not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

10.3.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of a built-infunction objectF takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:

  1. Let callerContext be therunning execution context.
  2. If callerContext is not already suspended, suspend callerContext.
  3. Let calleeContext be a newexecution context.
  4. Set the Function of calleeContext to F.
  5. Let calleeRealm be F.[[Realm]].
  6. Set theRealmof calleeContext to calleeRealm.
  7. Set the ScriptOrModule of calleeContext tonull.
  8. Perform any necessaryimplementation-definedinitialization of calleeContext.
  9. Push calleeContext onto theexecution context stack; calleeContext is now therunning execution context.
  10. Let result be theCompletion Recordthat is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is thethisvalue, argumentsList provides the named parameters, and the NewTarget value isundefined.
  11. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
  12. Return result.
Note

When calleeContext is removed from theexecution context stackit must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.

10.3.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of a built-infunction objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). The steps performed are the same as [[Call]] (see10.3.1) except that step10is replaced by:

  1. Let result be theCompletion Recordthat is the result of evaluating F in a manner that conforms to the specification of F. Thethisvalue is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.

10.3.3 CreateBuiltinFunction ( behaviour, length, name, additionalInternalSlotsList [ , realm [ , prototype [ , prefix ] ] ] )

The abstract operation CreateBuiltinFunction takes arguments behaviour, length (a non-negativeintegeror +∞), name (a property key), and additionalInternalSlotsList (aListof names of internal slots) and optional arguments realm (aRealm Record), prototype (an Object ornull), and prefix (a String). additionalInternalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-infunction object. It performs the following steps when called:

  1. Assert: behaviour is either anAbstract Closure, a set of algorithm steps, or some other definition of a function's behaviour provided in this specification.
  2. If realm is not present, set realm tothe current Realm Record.
  3. Assert: realm is aRealm Record.
  4. If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
  5. Let internalSlotsList be aListcontaining the names of all the internal slots that10.3requires for the built-infunction objectthat is about to be created.
  6. Append to internalSlotsList the elements of additionalInternalSlotsList.
  7. Let func be a new built-infunction objectthat, when called, performs the action described by behaviour using the provided arguments as the values of the corresponding parameters specified by behaviour. The newfunction objecthas internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
  8. Set func.[[Prototype]] to prototype.
  9. Set func.[[Extensible]] totrue.
  10. Set func.[[Realm]] to realm.
  11. Set func.[[InitialName]] tonull.
  12. Perform ! SetFunctionLength(func, length).
  13. If prefix is not present, then
    1. Perform ! SetFunctionName(func, name).
  14. Else,
    1. Perform ! SetFunctionName(func, name, prefix).
  15. Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.

10.4 Built-in Exotic Object Internal Methods and Slots

This specification defines several kinds of built-in exotic objects. These objects generally behave similar to ordinary objects except for a few specific situations. The following exotic objects use theordinary objectinternal methods except where it is explicitly specified otherwise below:

10.4.1 Bound Function Exotic Objects

Abound function exotic objectis anexotic objectthat wraps anotherfunction object. Abound function exotic objectis callable (it has a [[Call]] internal method and may have a [[Construct]] internal method). Calling abound function exotic objectgenerally results in a call of its wrapped function.

An object is a bound function exotic object if its [[Call]] and (if applicable) [[Construct]] internal methods use the following implementations, and its other essential internal methods use the definitions found in10.1. These methods are installed inBoundFunctionCreate.

Bound function exotic objects do not have the internal slots of ECMAScript function objects listed inTable 33. Instead they have the internal slots listed inTable 34, in addition to [[Prototype]] and [[Extensible]].

Table 34: Internal Slots of Bound Function Exotic Objects
Internal SlotTypeDescription
[[BoundTargetFunction]]Callable ObjectThe wrappedfunction object.
[[BoundThis]]AnyThe value that is always passed as thethisvalue when calling the wrapped function.
[[BoundArguments]]Listof AnyA list of values whose elements are used as the first arguments to any call to the wrapped function.

10.4.1.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of abound function exotic objectF takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:

  1. Let target be F.[[BoundTargetFunction]].
  2. Let boundThis be F.[[BoundThis]].
  3. Let boundArgs be F.[[BoundArguments]].
  4. Let args be thelist-concatenationof boundArgs and argumentsList.
  5. Return ? Call(target, boundThis, args).

10.4.1.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of abound function exotic objectF takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:

  1. Let target be F.[[BoundTargetFunction]].
  2. Assert:IsConstructor(target) istrue.
  3. Let boundArgs be F.[[BoundArguments]].
  4. Let args be thelist-concatenationof boundArgs and argumentsList.
  5. IfSameValue(F, newTarget) istrue, set newTarget to target.
  6. Return ? Construct(target, args, newTarget).

10.4.1.3 BoundFunctionCreate ( targetFunction, boundThis, boundArgs )

The abstract operation BoundFunctionCreate takes arguments targetFunction, boundThis, and boundArgs. It is used to specify the creation of new bound function exotic objects. It performs the following steps when called:

  1. Assert:Type(targetFunction) is Object.
  2. Let proto be ? targetFunction.[[GetPrototypeOf]]().
  3. Let internalSlotsList be the internal slots listed inTable 34, plus [[Prototype]] and [[Extensible]].
  4. Let obj be ! MakeBasicObject(internalSlotsList).
  5. Set obj.[[Prototype]] to proto.
  6. Set obj.[[Call]] as described in10.4.1.1.
  7. IfIsConstructor(targetFunction) istrue, then
    1. Set obj.[[Construct]] as described in10.4.1.2.
  8. Set obj.[[BoundTargetFunction]] to targetFunction.
  9. Set obj.[[BoundThis]] to boundThis.
  10. Set obj.[[BoundArguments]] to boundArgs.
  11. Return obj.

10.4.2 Array Exotic Objects

An Array object is anexotic objectthat gives special treatment toarray indexproperty keys (see6.1.7). A property whoseproperty nameis anarray indexis also called an element. Every Array object has a non-configurable"length"property whose value is always a non-negativeintegral Numberwhosemathematical valueis less than 232. The value of the"length"property is numerically greater than the name of every own property whose name is anarray index; whenever an own property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name is anarray index, the value of the"length"property is changed, if necessary, to be one more than the numeric value of thatarray index; and whenever the value of the"length"property is changed, every own property whose name is anarray indexwhose value is not smaller than the new length is deleted. This constraint applies only to own properties of an Array object and is unaffected by"length"orarray indexproperties that may be inherited from its prototypes.

Note

A Stringproperty nameP is an array index if and only ifToString(ToUint32(P)) equals P andToUint32(P) is not the same value as𝔽(232 - 1).

An object is an Array exotic object (or simply, an Array object) if its [[DefineOwnProperty]] internal method uses the following implementation, and its other essential internal methods use the definitions found in10.1. These methods are installed inArrayCreate.

10.4.2.1 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of anArray exotic objectA takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. If P is"length", then
    1. Return ? ArraySetLength(A, Desc).
  3. Else if P is anarray index, then
    1. Let oldLenDesc beOrdinaryGetOwnProperty(A,"length").
    2. Assert: ! IsDataDescriptor(oldLenDesc) istrue.
    3. Assert: oldLenDesc.[[Configurable]] isfalse.
    4. Let oldLen be oldLenDesc.[[Value]].
    5. Assert: oldLen is a non-negativeintegral Number.
    6. Let index be ! ToUint32(P).
    7. If indexoldLen and oldLenDesc.[[Writable]] isfalse, returnfalse.
    8. Let succeeded be ! OrdinaryDefineOwnProperty(A, P, Desc).
    9. If succeeded isfalse, returnfalse.
    10. If indexoldLen, then
      1. Set oldLenDesc.[[Value]] to index +1𝔽.
      2. Set succeeded toOrdinaryDefineOwnProperty(A,"length", oldLenDesc).
      3. Assert: succeeded istrue.
    11. Returntrue.
  4. ReturnOrdinaryDefineOwnProperty(A, P, Desc).

10.4.2.2 ArrayCreate ( length [ , proto ] )

The abstract operation ArrayCreate takes argument length (a non-negativeinteger) and optional argument proto. It is used to specify the creation of new Array exotic objects. It performs the following steps when called:

  1. If length > 232 - 1, throw aRangeErrorexception.
  2. If proto is not present, set proto to%Array.prototype%.
  3. Let A be ! MakeBasicObject(« [[Prototype]], [[Extensible]] »).
  4. Set A.[[Prototype]] to proto.
  5. Set A.[[DefineOwnProperty]] as specified in10.4.2.1.
  6. Perform ! OrdinaryDefineOwnProperty(A,"length", PropertyDescriptor { [[Value]]:𝔽(length), [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  7. Return A.

10.4.2.3 ArraySpeciesCreate ( originalArray, length )

The abstract operation ArraySpeciesCreate takes arguments originalArray and length (a non-negativeinteger). It is used to specify the creation of a new Array object using aconstructorfunction that is derived from originalArray. It performs the following steps when called:

  1. Let isArray be ? IsArray(originalArray).
  2. If isArray isfalse, return ? ArrayCreate(length).
  3. Let C be ? Get(originalArray,"constructor").
  4. IfIsConstructor(C) istrue, then
    1. Let thisRealm bethe current Realm Record.
    2. Let realmC be ? GetFunctionRealm(C).
    3. If thisRealm and realmC are not the sameRealm Record, then
      1. IfSameValue(C, realmC.[[Intrinsics]].[[%Array%]]) istrue, set C toundefined.
  5. IfType(C) is Object, then
    1. Set C to ? Get(C,@@species).
    2. If C isnull, set C toundefined.
  6. If C isundefined, return ? ArrayCreate(length).
  7. IfIsConstructor(C) isfalse, throw aTypeErrorexception.
  8. Return ? Construct(C, «𝔽(length) »).
Note

If originalArray was created using the standard built-in Arrayconstructorfor arealmthat is not therealmof therunning execution context, then a new Array is created using therealmof therunning execution context. This maintains compatibility with Web browsers that have historically had that behaviour for the Array.prototype methods that now are defined using ArraySpeciesCreate.

10.4.2.4 ArraySetLength ( A, Desc )

The abstract operation ArraySetLength takes arguments A (an Array object) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. If Desc.[[Value]] is absent, then
    1. ReturnOrdinaryDefineOwnProperty(A,"length", Desc).
  2. Let newLenDesc be a copy of Desc.
  3. Let newLen be ? ToUint32(Desc.[[Value]]).
  4. Let numberLen be ? ToNumber(Desc.[[Value]]).
  5. IfSameValueZero(newLen, numberLen) isfalse, throw aRangeErrorexception.
  6. Set newLenDesc.[[Value]] to newLen.
  7. Let oldLenDesc beOrdinaryGetOwnProperty(A,"length").
  8. Assert: ! IsDataDescriptor(oldLenDesc) istrue.
  9. Assert: oldLenDesc.[[Configurable]] isfalse.
  10. Let oldLen be oldLenDesc.[[Value]].
  11. If newLenoldLen, then
    1. ReturnOrdinaryDefineOwnProperty(A,"length", newLenDesc).
  12. If oldLenDesc.[[Writable]] isfalse, returnfalse.
  13. If newLenDesc.[[Writable]] is absent or has the valuetrue, let newWritable betrue.
  14. Else,
    1. NOTE: Setting the [[Writable]] attribute tofalseis deferred in case any elements cannot be deleted.
    2. Let newWritable befalse.
    3. Set newLenDesc.[[Writable]] totrue.
  15. Let succeeded be ! OrdinaryDefineOwnProperty(A,"length", newLenDesc).
  16. If succeeded isfalse, returnfalse.
  17. For each own property key P of A that is anarray index, whose numeric value is greater than or equal to newLen, in descending numeric index order, do
    1. Let deleteSucceeded be ! A.[[Delete]](P).
    2. If deleteSucceeded isfalse, then
      1. Set newLenDesc.[[Value]] to ! ToUint32(P) +1𝔽.
      2. If newWritable isfalse, set newLenDesc.[[Writable]] tofalse.
      3. Perform ! OrdinaryDefineOwnProperty(A,"length", newLenDesc).
      4. Returnfalse.
  18. If newWritable isfalse, then
    1. Set succeeded to ! OrdinaryDefineOwnProperty(A,"length", PropertyDescriptor { [[Writable]]:false}).
    2. Assert: succeeded istrue.
  19. Returntrue.
Note

In steps3and4, if Desc.[[Value]] is an object then its valueOf method is called twice. This is legacy behaviour that was specified with this effect starting with the 2nd Edition of this specification.

10.4.3 String Exotic Objects

A String object is anexotic objectthat encapsulates a String value and exposes virtualinteger-indexed data properties corresponding to the individual code unit elements of the String value. String exotic objects always have adata propertynamed"length"whose value is the number of code unit elements in the encapsulated String value. Both the code unit data properties and the"length"property are non-writable and non-configurable.

An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal methods use the following implementations, and its other essential internal methods use the definitions found in10.1. These methods are installed inStringCreate.

String exotic objects have the same internal slots as ordinary objects. They also have a [[StringData]] internal slot.

10.4.3.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of aString exotic objectS takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let desc beOrdinaryGetOwnProperty(S, P).
  3. If desc is notundefined, return desc.
  4. Return ! StringGetOwnProperty(S, P).

10.4.3.2 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of aString exotic objectS takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let stringDesc be ! StringGetOwnProperty(S, P).
  3. If stringDesc is notundefined, then
    1. Let extensible be S.[[Extensible]].
    2. Return ! IsCompatiblePropertyDescriptor(extensible, Desc, stringDesc).
  4. Return ! OrdinaryDefineOwnProperty(S, P, Desc).

10.4.3.3 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of aString exotic objectO takes no arguments. It performs the following steps when called:

  1. Let keys be a new emptyList.
  2. Let str be O.[[StringData]].
  3. Assert:Type(str) is String.
  4. Let len be the length of str.
  5. For eachintegeri starting with 0 such that i < len, in ascending order, do
    1. Add ! ToString(𝔽(i)) as the last element of keys.
  6. For each own property key P of O such that P is anarray indexand ! ToIntegerOrInfinity(P) ≥ len, in ascending numeric index order, do
    1. Add P as the last element of keys.
  7. For each own property key P of O such thatType(P) is String and P is not anarray index, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  8. For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  9. Return keys.

10.4.3.4 StringCreate ( value, prototype )

The abstract operation StringCreate takes arguments value (a String) and prototype. It is used to specify the creation of new String exotic objects. It performs the following steps when called:

  1. Let S be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
  2. Set S.[[Prototype]] to prototype.
  3. Set S.[[StringData]] to value.
  4. Set S.[[GetOwnProperty]] as specified in10.4.3.1.
  5. Set S.[[DefineOwnProperty]] as specified in10.4.3.2.
  6. Set S.[[OwnPropertyKeys]] as specified in10.4.3.3.
  7. Let length be the number of code unit elements in value.
  8. Perform ! DefinePropertyOrThrow(S,"length", PropertyDescriptor { [[Value]]:𝔽(length), [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}).
  9. Return S.

10.4.3.5 StringGetOwnProperty ( S, P )

The abstract operation StringGetOwnProperty takes arguments S and P. It performs the following steps when called:

  1. Assert: S is an Object that has a [[StringData]] internal slot.
  2. Assert:IsPropertyKey(P) istrue.
  3. IfType(P) is not String, returnundefined.
  4. Let index be ! CanonicalNumericIndexString(P).
  5. If index isundefined, returnundefined.
  6. IfIsIntegralNumber(index) isfalse, returnundefined.
  7. If index is-0𝔽, returnundefined.
  8. Let str be S.[[StringData]].
  9. Assert:Type(str) is String.
  10. Let len be the length of str.
  11. If(index) < 0 or len(index), returnundefined.
  12. Let resultStr be the String value of length 1, containing one code unit from str, specifically the code unit at index(index).
  13. Return the PropertyDescriptor { [[Value]]: resultStr, [[Writable]]:false, [[Enumerable]]:true, [[Configurable]]:false}.

10.4.4 Arguments Exotic Objects

Most ECMAScript functions make an arguments object available to their code. Depending upon the characteristics of the function definition, its arguments object is either anordinary objector anarguments exotic object. Anarguments exotic objectis anexotic objectwhosearray indexproperties map to the formal parameters bindings of an invocation of its associated ECMAScript function.

An object is an arguments exotic object if its internal methods use the following implementations, with the ones not specified here using those found in10.1. These methods are installed inCreateMappedArgumentsObject.

Note 1

WhileCreateUnmappedArgumentsObjectis grouped into this clause, it creates anordinary object, not anarguments exotic object.

Arguments exotic objects have the same internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by Object.prototype.toString (20.1.3.6) to identify them as such.

Note 2

Theinteger-indexed data properties of anarguments exotic objectwhose numeric name values are less than the number of formal parameters of the correspondingfunction objectinitially share their values with the corresponding argument bindings in the function'sexecution context. This means that changing the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if such a property is deleted and then redefined or if the property is changed into anaccessor property. If the arguments object is anordinary object, the values of its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage between the property values and the formal parameter values.

Note 3

The ParameterMap object and its property values are used as a device for specifying the arguments object correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does not need to actually create or use such objects to implement the specified semantics.

Note 4

Ordinary arguments objects define a non-configurableaccessor propertynamed"callee"which throws aTypeErrorexception on access. The"callee"property has a more specific meaning for arguments exotic objects, which are created only for some class of non-strict functions. The definition of this property in the ordinary variant exists to ensure that it is not defined in any other manner by conforming ECMAScript implementations.

Note 5

ECMAScript implementations of arguments exotic objects have historically contained anaccessor propertynamed"caller". Prior to ECMAScript 2017, this specification included the definition of a throwing"caller"property on ordinary arguments objects. Since implementations do not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing"caller"accessor.

10.4.4.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of anarguments exotic objectargs takes argument P (a property key). It performs the following steps when called:

  1. Let desc beOrdinaryGetOwnProperty(args, P).
  2. If desc isundefined, return desc.
  3. Let map be args.[[ParameterMap]].
  4. Let isMapped be ! HasOwnProperty(map, P).
  5. If isMapped istrue, then
    1. Set desc.[[Value]] toGet(map, P).
  6. Return desc.

10.4.4.2 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of anarguments exotic objectargs takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped beHasOwnProperty(map, P).
  3. Let newArgDesc be Desc.
  4. If isMapped istrueandIsDataDescriptor(Desc) istrue, then
    1. If Desc.[[Value]] is not present and Desc.[[Writable]] is present and its value isfalse, then
      1. Set newArgDesc to a copy of Desc.
      2. Set newArgDesc.[[Value]] toGet(map, P).
  5. Let allowed be ? OrdinaryDefineOwnProperty(args, P, newArgDesc).
  6. If allowed isfalse, returnfalse.
  7. If isMapped istrue, then
    1. IfIsAccessorDescriptor(Desc) istrue, then
      1. Call map.[[Delete]](P).
    2. Else,
      1. If Desc.[[Value]] is present, then
        1. Let setStatus beSet(map, P, Desc.[[Value]],false).
        2. Assert: setStatus istruebecause formal parameters mapped by argument objects are always writable.
      2. If Desc.[[Writable]] is present and its value isfalse, then
        1. Call map.[[Delete]](P).
  8. Returntrue.

10.4.4.3 [[Get]] ( P, Receiver )

The [[Get]] internal method of anarguments exotic objectargs takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped be ! HasOwnProperty(map, P).
  3. If isMapped isfalse, then
    1. Return ? OrdinaryGet(args, P, Receiver).
  4. Else,
    1. Assert: map contains a formal parameter mapping for P.
    2. ReturnGet(map, P).

10.4.4.4 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of anarguments exotic objectargs takes arguments P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. IfSameValue(args, Receiver) isfalse, then
    1. Let isMapped befalse.
  2. Else,
    1. Let map be args.[[ParameterMap]].
    2. Let isMapped be ! HasOwnProperty(map, P).
  3. If isMapped istrue, then
    1. Let setStatus beSet(map, P, V,false).
    2. Assert: setStatus istruebecause formal parameters mapped by argument objects are always writable.
  4. Return ? OrdinarySet(args, P, V, Receiver).

10.4.4.5 [[Delete]] ( P )

The [[Delete]] internal method of anarguments exotic objectargs takes argument P (a property key). It performs the following steps when called:

  1. Let map be args.[[ParameterMap]].
  2. Let isMapped be ! HasOwnProperty(map, P).
  3. Let result be ? OrdinaryDelete(args, P).
  4. If result istrueand isMapped istrue, then
    1. Call map.[[Delete]](P).
  5. Return result.

10.4.4.6 CreateUnmappedArgumentsObject ( argumentsList )

The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList. It performs the following steps when called:

  1. Let len be the number of elements in argumentsList.
  2. Let obj be ! OrdinaryObjectCreate(%Object.prototype%, « [[ParameterMap]] »).
  3. Set obj.[[ParameterMap]] toundefined.
  4. PerformDefinePropertyOrThrow(obj,"length", PropertyDescriptor { [[Value]]:𝔽(len), [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}).
  5. Let index be 0.
  6. Repeat, while index < len,
    1. Let val be argumentsList[index].
    2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
    3. Set index to index + 1.
  7. Perform ! DefinePropertyOrThrow(obj,@@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}).
  8. Perform ! DefinePropertyOrThrow(obj,"callee", PropertyDescriptor { [[Get]]:%ThrowTypeError%, [[Set]]:%ThrowTypeError%, [[Enumerable]]:false, [[Configurable]]:false}).
  9. Return obj.

10.4.4.7 CreateMappedArgumentsObject ( func, formals, argumentsList, env )

The abstract operation CreateMappedArgumentsObject takes arguments func (an Object), formals (aParse Node), argumentsList (aList), and env (anEnvironment Record). It performs the following steps when called:

  1. Assert: formals does not contain a rest parameter, any binding patterns, or any initializers. It may contain duplicate identifiers.
  2. Let len be the number of elements in argumentsList.
  3. Let obj be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[ParameterMap]] »).
  4. Set obj.[[GetOwnProperty]] as specified in10.4.4.1.
  5. Set obj.[[DefineOwnProperty]] as specified in10.4.4.2.
  6. Set obj.[[Get]] as specified in10.4.4.3.
  7. Set obj.[[Set]] as specified in10.4.4.4.
  8. Set obj.[[Delete]] as specified in10.4.4.5.
  9. Set obj.[[Prototype]] to%Object.prototype%.
  10. Let map be ! OrdinaryObjectCreate(null).
  11. Set obj.[[ParameterMap]] to map.
  12. Let parameterNames be theBoundNamesof formals.
  13. Let numberOfParameters be the number of elements in parameterNames.
  14. Let index be 0.
  15. Repeat, while index < len,
    1. Let val be argumentsList[index].
    2. Perform ! CreateDataPropertyOrThrow(obj, ! ToString(𝔽(index)), val).
    3. Set index to index + 1.
  16. Perform ! DefinePropertyOrThrow(obj,"length", PropertyDescriptor { [[Value]]:𝔽(len), [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}).
  17. Let mappedNames be a new emptyList.
  18. Set index to numberOfParameters - 1.
  19. Repeat, while index ≥ 0,
    1. Let name be parameterNames[index].
    2. If name is not an element of mappedNames, then
      1. Add name as an element of the list mappedNames.
      2. If index < len, then
        1. Let g beMakeArgGetter(name, env).
        2. Let p beMakeArgSetter(name, env).
        3. Perform map.[[DefineOwnProperty]](!ToString(𝔽(index)), PropertyDescriptor { [[Set]]: p, [[Get]]: g, [[Enumerable]]:false, [[Configurable]]:true}).
    3. Set index to index - 1.
  20. Perform ! DefinePropertyOrThrow(obj,@@iterator, PropertyDescriptor { [[Value]]: %Array.prototype.values%, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}).
  21. Perform ! DefinePropertyOrThrow(obj,"callee", PropertyDescriptor { [[Value]]: func, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}).
  22. Return obj.

10.4.4.7.1 MakeArgGetter ( name, env )

The abstract operation MakeArgGetter takes arguments name (a String) and env (anEnvironment Record). It creates a built-infunction objectthat when executed returns the value bound for name in env. It performs the following steps when called:

  1. Let getterClosure be a newAbstract Closurewith no parameters that captures name and env and performs the following steps when called:
    1. Return env.GetBindingValue(name,false).
  2. Let getter be ! CreateBuiltinFunction(getterClosure, 0,"", « »).
  3. NOTE: getter is never directly accessible to ECMAScript code.
  4. Return getter.

10.4.4.7.2 MakeArgSetter ( name, env )

The abstract operation MakeArgSetter takes arguments name (a String) and env (anEnvironment Record). It creates a built-infunction objectthat when executed sets the value bound for name in env. It performs the following steps when called:

  1. Let setterClosure be a newAbstract Closurewith parameters (value) that captures name and env and performs the following steps when called:
    1. Return env.SetMutableBinding(name, value,false).
  2. Let setter be ! CreateBuiltinFunction(setterClosure, 1,"", « »).
  3. NOTE: setter is never directly accessible to ECMAScript code.
  4. Return setter.

10.4.5 Integer-Indexed Exotic Objects

AnInteger-Indexed exotic objectis anexotic objectthat performs special handling ofinteger indexproperty keys.

Integer-Indexed exotic objectshave the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.

An object is an Integer-Indexed exotic object if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in10.1. These methods are installed byIntegerIndexedObjectCreate.

10.4.5.1 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of anInteger-Indexed exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Assert: O is anInteger-Indexed exotic object.
  3. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, then
      1. Let value be ! IntegerIndexedElementGet(O, numericIndex).
      2. If value isundefined, returnundefined.
      3. Return the PropertyDescriptor { [[Value]]: value, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:true}.
  4. ReturnOrdinaryGetOwnProperty(O, P).

10.4.5.2 [[HasProperty]] ( P )

The [[HasProperty]] internal method of anInteger-Indexed exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Assert: O is anInteger-Indexed exotic object.
  3. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, return ! IsValidIntegerIndex(O, numericIndex).
  4. Return ? OrdinaryHasProperty(O, P).

10.4.5.3 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of anInteger-Indexed exotic objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Assert: O is anInteger-Indexed exotic object.
  3. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, then
      1. If ! IsValidIntegerIndex(O, numericIndex) isfalse, returnfalse.
      2. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] isfalse, returnfalse.
      3. If Desc has an [[Enumerable]] field and if Desc.[[Enumerable]] isfalse, returnfalse.
      4. If ! IsAccessorDescriptor(Desc) istrue, returnfalse.
      5. If Desc has a [[Writable]] field and if Desc.[[Writable]] isfalse, returnfalse.
      6. If Desc has a [[Value]] field, perform ? IntegerIndexedElementSet(O, numericIndex, Desc.[[Value]]).
      7. Returntrue.
  4. Return ! OrdinaryDefineOwnProperty(O, P, Desc).

10.4.5.4 [[Get]] ( P, Receiver )

The [[Get]] internal method of anInteger-Indexed exotic objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, then
      1. Return ! IntegerIndexedElementGet(O, numericIndex).
  3. Return ? OrdinaryGet(O, P, Receiver).

10.4.5.5 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of anInteger-Indexed exotic objectO takes arguments P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, then
      1. Perform ? IntegerIndexedElementSet(O, numericIndex, V).
      2. Returntrue.
  3. Return ? OrdinarySet(O, P, V, Receiver).

10.4.5.6 [[Delete]] ( P )

The [[Delete]] internal method of anInteger-Indexed exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Assert: O is anInteger-Indexed exotic object.
  3. IfType(P) is String, then
    1. Let numericIndex be ! CanonicalNumericIndexString(P).
    2. If numericIndex is notundefined, then
      1. If ! IsValidIntegerIndex(O, numericIndex) isfalse, returntrue; else returnfalse.
  4. Return ? OrdinaryDelete(O, P).

10.4.5.7 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of anInteger-Indexed exotic objectO takes no arguments. It performs the following steps when called:

  1. Let keys be a new emptyList.
  2. Assert: O is anInteger-Indexed exotic object.
  3. IfIsDetachedBuffer(O.[[ViewedArrayBuffer]]) isfalse, then
    1. For eachintegeri starting with 0 such that i < O.[[ArrayLength]], in ascending order, do
      1. Add ! ToString(𝔽(i)) as the last element of keys.
  4. For each own property key P of O such thatType(P) is String and P is not aninteger index, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  5. For each own property key P of O such thatType(P) is Symbol, in ascending chronological order of property creation, do
    1. Add P as the last element of keys.
  6. Return keys.

10.4.5.8 IntegerIndexedObjectCreate ( prototype )

The abstract operation IntegerIndexedObjectCreate takes argument prototype. It is used to specify the creation of newInteger-Indexed exotic objects. It performs the following steps when called:

  1. Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
  2. Let A be ! MakeBasicObject(internalSlotsList).
  3. Set A.[[GetOwnProperty]] as specified in10.4.5.1.
  4. Set A.[[HasProperty]] as specified in10.4.5.2.
  5. Set A.[[DefineOwnProperty]] as specified in10.4.5.3.
  6. Set A.[[Get]] as specified in10.4.5.4.
  7. Set A.[[Set]] as specified in10.4.5.5.
  8. Set A.[[Delete]] as specified in10.4.5.6.
  9. Set A.[[OwnPropertyKeys]] as specified in10.4.5.7.
  10. Set A.[[Prototype]] to prototype.
  11. Return A.

10.4.5.9 IsValidIntegerIndex ( O, index )

The abstract operation IsValidIntegerIndex takes arguments O and index (a Number). It performs the following steps when called:

  1. Assert: O is anInteger-Indexed exotic object.
  2. IfIsDetachedBuffer(O.[[ViewedArrayBuffer]]) istrue, returnfalse.
  3. If ! IsIntegralNumber(index) isfalse, returnfalse.
  4. If index is-0𝔽, returnfalse.
  5. If(index) < 0 or(index) ≥ O.[[ArrayLength]], returnfalse.
  6. Returntrue.

10.4.5.10 IntegerIndexedElementGet ( O, index )

The abstract operation IntegerIndexedElementGet takes arguments O and index (a Number). It performs the following steps when called:

  1. Assert: O is anInteger-Indexed exotic object.
  2. If ! IsValidIntegerIndex(O, index) isfalse, returnundefined.
  3. Let offset be O.[[ByteOffset]].
  4. Let arrayTypeName be the String value of O.[[TypedArrayName]].
  5. Let elementSize be the Element Size value specified inTable 63for arrayTypeName.
  6. Let indexedPosition be ((index) × elementSize) + offset.
  7. Let elementType be the Element Type value inTable 63for arrayTypeName.
  8. ReturnGetValueFromBuffer(O.[[ViewedArrayBuffer]], indexedPosition, elementType,true,Unordered).

10.4.5.11 IntegerIndexedElementSet ( O, index, value )

The abstract operation IntegerIndexedElementSet takes arguments O, index (a Number), and value. It performs the following steps when called:

  1. Assert: O is anInteger-Indexed exotic object.
  2. If O.[[ContentType]] isBigInt, let numValue be ? ToBigInt(value).
  3. Otherwise, let numValue be ? ToNumber(value).
  4. If ! IsValidIntegerIndex(O, index) istrue, then
    1. Let offset be O.[[ByteOffset]].
    2. Let arrayTypeName be the String value of O.[[TypedArrayName]].
    3. Let elementSize be the Element Size value specified inTable 63for arrayTypeName.
    4. Let indexedPosition be ((index) × elementSize) + offset.
    5. Let elementType be the Element Type value inTable 63for arrayTypeName.
    6. PerformSetValueInBuffer(O.[[ViewedArrayBuffer]], indexedPosition, elementType, numValue,true,Unordered).
  5. ReturnNormalCompletion(undefined).
Note

This operation always appears to succeed, but it has no effect when attempting to write past the end of a TypedArray or to a TypedArray which is backed by a detached ArrayBuffer.

10.4.6 Module Namespace Exotic Objects

Amodule namespace exotic objectis anexotic objectthat exposes the bindings exported from an ECMAScriptModule(See16.2.3). There is a one-to-one correspondence between the String-keyed own properties of amodule namespace exotic objectand the binding names exported by theModule. The exported bindings include any bindings that are indirectly exported using export * export items. Each String-valued own property key is theStringValueof the corresponding exported binding name. These are the only String-keyed properties of amodule namespace exotic object. Each such property has the attributes { [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:false}. Module namespace exotic objects are not extensible.

An object is a module namespace exotic object if its [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its other essential internal methods use the definitions found in10.1. These methods are installed byModuleNamespaceCreate.

Module namespace exotic objects have the internal slots defined inTable 35.

Table 35: Internal Slots of Module Namespace Exotic Objects
Internal SlotTypeDescription
[[Module]]Module RecordTheModule Recordwhose exports this namespace exposes.
[[Exports]]Listof StringAListwhose elements are the String values of the exported names exposed as own properties of this object. The list is ordered as if an Array of those String values had been sorted using %Array.prototype.sort% usingundefinedas comparefn.
[[Prototype]]NullThis slot always contains the valuenull(see10.4.6.1).

Module namespace exotic objects provide alternative definitions for all of the internal methods except [[GetPrototypeOf]], which behaves as defined in10.1.1.

10.4.6.1 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of amodule namespace exotic objectO takes argument V (an Object ornull). It performs the following steps when called:

  1. Return ? SetImmutablePrototype(O, V).

10.4.6.2 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of amodule namespace exotic objecttakes no arguments. It performs the following steps when called:

  1. Returnfalse.

10.4.6.3 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of amodule namespace exotic objecttakes no arguments. It performs the following steps when called:

  1. Returntrue.

10.4.6.4 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of amodule namespace exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. IfType(P) is Symbol, returnOrdinaryGetOwnProperty(O, P).
  2. Let exports be O.[[Exports]].
  3. If P is not an element of exports, returnundefined.
  4. Let value be ? O.[[Get]](P, O).
  5. Return PropertyDescriptor { [[Value]]: value, [[Writable]]:true, [[Enumerable]]:true, [[Configurable]]:false}.

10.4.6.5 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of amodule namespace exotic objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. IfType(P) is Symbol, returnOrdinaryDefineOwnProperty(O, P, Desc).
  2. Let current be ? O.[[GetOwnProperty]](P).
  3. If current isundefined, returnfalse.
  4. If Desc.[[Configurable]] is present and has valuetrue, returnfalse.
  5. If Desc.[[Enumerable]] is present and has valuefalse, returnfalse.
  6. If ! IsAccessorDescriptor(Desc) istrue, returnfalse.
  7. If Desc.[[Writable]] is present and has valuefalse, returnfalse.
  8. If Desc.[[Value]] is present, returnSameValue(Desc.[[Value]], current.[[Value]]).
  9. Returntrue.

10.4.6.6 [[HasProperty]] ( P )

The [[HasProperty]] internal method of amodule namespace exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. IfType(P) is Symbol, returnOrdinaryHasProperty(O, P).
  2. Let exports be O.[[Exports]].
  3. If P is an element of exports, returntrue.
  4. Returnfalse.

10.4.6.7 [[Get]] ( P, Receiver )

The [[Get]] internal method of amodule namespace exotic objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. IfType(P) is Symbol, then
    1. Return ? OrdinaryGet(O, P, Receiver).
  3. Let exports be O.[[Exports]].
  4. If P is not an element of exports, returnundefined.
  5. Let m be O.[[Module]].
  6. Let binding be ! m.ResolveExport(P).
  7. Assert: binding is aResolvedBinding Record.
  8. Let targetModule be binding.[[Module]].
  9. Assert: targetModule is notundefined.
  10. If binding.[[BindingName]] is"*namespace*", then
    1. Return ? GetModuleNamespace(targetModule).
  11. Let targetEnv be targetModule.[[Environment]].
  12. If targetEnv isundefined, throw aReferenceErrorexception.
  13. Return ? targetEnv.GetBindingValue(binding.[[BindingName]],true).
Note

ResolveExport is side-effect free. Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result. An implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of eachmodule namespace exotic object.

10.4.6.8 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of amodule namespace exotic objecttakes arguments P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Returnfalse.

10.4.6.9 [[Delete]] ( P )

The [[Delete]] internal method of amodule namespace exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. IfType(P) is Symbol, then
    1. Return ? OrdinaryDelete(O, P).
  3. Let exports be O.[[Exports]].
  4. If P is an element of exports, returnfalse.
  5. Returntrue.

10.4.6.10 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of amodule namespace exotic objectO takes no arguments. It performs the following steps when called:

  1. Let exports be O.[[Exports]].
  2. Let symbolKeys be ! OrdinaryOwnPropertyKeys(O).
  3. Return thelist-concatenationof exports and symbolKeys.

10.4.6.11 ModuleNamespaceCreate ( module, exports )

The abstract operation ModuleNamespaceCreate takes arguments module and exports. It is used to specify the creation of new module namespace exotic objects. It performs the following steps when called:

  1. Assert: module is aModule Record.
  2. Assert: module.[[Namespace]] isundefined.
  3. Assert: exports is aListof String values.
  4. Let internalSlotsList be the internal slots listed inTable 35.
  5. Let M be ! MakeBasicObject(internalSlotsList).
  6. Set M's essential internal methods to the definitions specified in10.4.6.
  7. Set M.[[Prototype]] tonull.
  8. Set M.[[Module]] to module.
  9. Let sortedExports be aListwhose elements are the elements of exports ordered as if an Array of the same values had been sorted using %Array.prototype.sort% usingundefinedas comparefn.
  10. Set M.[[Exports]] to sortedExports.
  11. Create own properties of M corresponding to the definitions in28.3.
  12. Set module.[[Namespace]] to M.
  13. Return M.

10.4.7 Immutable Prototype Exotic Objects

Animmutable prototype exotic objectis anexotic objectthat has a [[Prototype]] internal slot that will not change once it is initialized.

An object is an immutable prototype exotic object if its [[SetPrototypeOf]] internal method uses the following implementation. (Its other essential internal methods may use any implementation, depending on the specificimmutable prototype exotic objectin question.)

Note

Unlike other exotic objects, there is not a dedicated creation abstract operation provided for immutable prototype exotic objects. This is because they are only used by%Object.prototype%and byhostenvironments, and inhostenvironments, the relevant objects are potentially exotic in other ways and thus need their own dedicated creation operation.

10.4.7.1 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of animmutable prototype exotic objectO takes argument V (an Object ornull). It performs the following steps when called:

  1. Return ? SetImmutablePrototype(O, V).

10.4.7.2 SetImmutablePrototype ( O, V )

The abstract operation SetImmutablePrototype takes arguments O and V. It performs the following steps when called:

  1. Assert: EitherType(V) is Object orType(V) is Null.
  2. Let current be ? O.[[GetPrototypeOf]]().
  3. IfSameValue(V, current) istrue, returntrue.
  4. Returnfalse.

10.5 Proxy Object Internal Methods and Internal Slots

A proxy object is anexotic objectwhose essential internal methods are partially implemented using ECMAScript code. Every proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object, called the proxy's handler object, ornull. Methods (seeTable 36) of a handler object may be used to augment the implementation for one or more of the proxy object's internal methods. Every proxy object also has an internal slot called [[ProxyTarget]] whose value is either an object or thenullvalue. This object is called the proxy's target object.

An object is a Proxy exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal methods are installed inProxyCreate.

Table 36: Proxy Handler Methods
Internal MethodHandler Method
[[GetPrototypeOf]]getPrototypeOf
[[SetPrototypeOf]]setPrototypeOf
[[IsExtensible]]isExtensible
[[PreventExtensions]]preventExtensions
[[GetOwnProperty]]getOwnPropertyDescriptor
[[DefineOwnProperty]]defineProperty
[[HasProperty]]has
[[Get]]get
[[Set]]set
[[Delete]]deleteProperty
[[OwnPropertyKeys]]ownKeys
[[Call]]apply
[[Construct]]construct

When a handler method is called to provide the implementation of a proxy object internal method, the handler method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have a method corresponding to every essential internal method. Invoking an internal method on the proxy results in the invocation of the corresponding internal method on the proxy's target object if the handler object does not have a method corresponding to the internal trap.

The [[ProxyHandler]] and [[ProxyTarget]] internal slots of a proxy object are always initialized when the object is created and typically may not be modified. Some proxy objects are created in a manner that permits them to be subsequently revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set tonullcausing subsequent invocations of internal methods on that proxy object to throw aTypeErrorexception.

Because proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript code, it is possible to define a proxy object whose handler methods violates the invariants defined in6.1.7.3. Some of the internal method invariants defined in6.1.7.3are essential integrity invariants. These invariants are explicitly enforced by the proxy object internal methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible invariant violations.

In the following algorithm descriptions, assume O is an ECMAScript proxy object, P is a property key value, V is anyECMAScript language valueand Desc is aProperty Descriptorrecord.

10.5.1 [[GetPrototypeOf]] ( )

The [[GetPrototypeOf]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler,"getPrototypeOf").
  6. If trap isundefined, then
    1. Return ? target.[[GetPrototypeOf]]().
  7. Let handlerProto be ? Call(trap, handler, « target »).
  8. IfType(handlerProto) is neither Object nor Null, throw aTypeErrorexception.
  9. Let extensibleTarget be ? IsExtensible(target).
  10. If extensibleTarget istrue, return handlerProto.
  11. Let targetProto be ? target.[[GetPrototypeOf]]().
  12. IfSameValue(handlerProto, targetProto) isfalse, throw aTypeErrorexception.
  13. Return handlerProto.
Note

[[GetPrototypeOf]] for proxy objects enforces the following invariants:

  • The result of [[GetPrototypeOf]] must be either an Object ornull.
  • If the target object is not extensible, [[GetPrototypeOf]] applied to the proxy object must return the same value as [[GetPrototypeOf]] applied to the proxy object's target object.

10.5.2 [[SetPrototypeOf]] ( V )

The [[SetPrototypeOf]] internal method of aProxy exotic objectO takes argument V (an Object ornull). It performs the following steps when called:

  1. Assert: EitherType(V) is Object orType(V) is Null.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"setPrototypeOf").
  7. If trap isundefined, then
    1. Return ? target.[[SetPrototypeOf]](V).
  8. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target, V »)).
  9. If booleanTrapResult isfalse, returnfalse.
  10. Let extensibleTarget be ? IsExtensible(target).
  11. If extensibleTarget istrue, returntrue.
  12. Let targetProto be ? target.[[GetPrototypeOf]]().
  13. IfSameValue(V, targetProto) isfalse, throw aTypeErrorexception.
  14. Returntrue.
Note

[[SetPrototypeOf]] for proxy objects enforces the following invariants:

  • The result of [[SetPrototypeOf]] is a Boolean value.
  • If the target object is not extensible, the argument value must be the same as the result of [[GetPrototypeOf]] applied to target object.

10.5.3 [[IsExtensible]] ( )

The [[IsExtensible]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler,"isExtensible").
  6. If trap isundefined, then
    1. Return ? IsExtensible(target).
  7. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target »)).
  8. Let targetResult be ? IsExtensible(target).
  9. IfSameValue(booleanTrapResult, targetResult) isfalse, throw aTypeErrorexception.
  10. Return booleanTrapResult.
Note

[[IsExtensible]] for proxy objects enforces the following invariants:

  • The result of [[IsExtensible]] is a Boolean value.
  • [[IsExtensible]] applied to the proxy object must return the same value as [[IsExtensible]] applied to the proxy object's target object with the same argument.

10.5.4 [[PreventExtensions]] ( )

The [[PreventExtensions]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler,"preventExtensions").
  6. If trap isundefined, then
    1. Return ? target.[[PreventExtensions]]().
  7. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target »)).
  8. If booleanTrapResult istrue, then
    1. Let extensibleTarget be ? IsExtensible(target).
    2. If extensibleTarget istrue, throw aTypeErrorexception.
  9. Return booleanTrapResult.
Note

[[PreventExtensions]] for proxy objects enforces the following invariants:

  • The result of [[PreventExtensions]] is a Boolean value.
  • [[PreventExtensions]] applied to the proxy object only returnstrueif [[IsExtensible]] applied to the proxy object's target object isfalse.

10.5.5 [[GetOwnProperty]] ( P )

The [[GetOwnProperty]] internal method of aProxy exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"getOwnPropertyDescriptor").
  7. If trap isundefined, then
    1. Return ? target.[[GetOwnProperty]](P).
  8. Let trapResultObj be ? Call(trap, handler, « target, P »).
  9. IfType(trapResultObj) is neither Object nor Undefined, throw aTypeErrorexception.
  10. Let targetDesc be ? target.[[GetOwnProperty]](P).
  11. If trapResultObj isundefined, then
    1. If targetDesc isundefined, returnundefined.
    2. If targetDesc.[[Configurable]] isfalse, throw aTypeErrorexception.
    3. Let extensibleTarget be ? IsExtensible(target).
    4. If extensibleTarget isfalse, throw aTypeErrorexception.
    5. Returnundefined.
  12. Let extensibleTarget be ? IsExtensible(target).
  13. Let resultDesc be ? ToPropertyDescriptor(trapResultObj).
  14. CallCompletePropertyDescriptor(resultDesc).
  15. Let valid beIsCompatiblePropertyDescriptor(extensibleTarget, resultDesc, targetDesc).
  16. If valid isfalse, throw aTypeErrorexception.
  17. If resultDesc.[[Configurable]] isfalse, then
    1. If targetDesc isundefinedor targetDesc.[[Configurable]] istrue, then
      1. Throw aTypeErrorexception.
    2. If resultDesc has a [[Writable]] field and resultDesc.[[Writable]] isfalse, then
      1. If targetDesc.[[Writable]] istrue, throw aTypeErrorexception.
  18. Return resultDesc.
Note

[[GetOwnProperty]] for proxy objects enforces the following invariants:

  • The result of [[GetOwnProperty]] must be either an Object orundefined.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if the target object is not extensible, unless it does not exist as an own property of the target object.
  • A property cannot be reported as existent, if the target object is not extensible, unless it exists as an own property of the target object.
  • A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object.
  • A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object.

10.5.6 [[DefineOwnProperty]] ( P, Desc )

The [[DefineOwnProperty]] internal method of aProxy exotic objectO takes arguments P (a property key) and Desc (aProperty Descriptor). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"defineProperty").
  7. If trap isundefined, then
    1. Return ? target.[[DefineOwnProperty]](P, Desc).
  8. Let descObj beFromPropertyDescriptor(Desc).
  9. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target, P, descObj »)).
  10. If booleanTrapResult isfalse, returnfalse.
  11. Let targetDesc be ? target.[[GetOwnProperty]](P).
  12. Let extensibleTarget be ? IsExtensible(target).
  13. If Desc has a [[Configurable]] field and if Desc.[[Configurable]] isfalse, then
    1. Let settingConfigFalse betrue.
  14. Else, let settingConfigFalse befalse.
  15. If targetDesc isundefined, then
    1. If extensibleTarget isfalse, throw aTypeErrorexception.
    2. If settingConfigFalse istrue, throw aTypeErrorexception.
  16. Else,
    1. IfIsCompatiblePropertyDescriptor(extensibleTarget, Desc, targetDesc) isfalse, throw aTypeErrorexception.
    2. If settingConfigFalse istrueand targetDesc.[[Configurable]] istrue, throw aTypeErrorexception.
    3. IfIsDataDescriptor(targetDesc) istrue, targetDesc.[[Configurable]] isfalse, and targetDesc.[[Writable]] istrue, then
      1. If Desc has a [[Writable]] field and Desc.[[Writable]] isfalse, throw aTypeErrorexception.
  17. Returntrue.
Note

[[DefineOwnProperty]] for proxy objects enforces the following invariants:

  • The result of [[DefineOwnProperty]] is a Boolean value.
  • A property cannot be added, if the target object is not extensible.
  • A property cannot be non-configurable, unless there exists a corresponding non-configurable own property of the target object.
  • A non-configurable property cannot be non-writable, unless there exists a corresponding non-configurable, non-writable own property of the target object.
  • If a property has a corresponding target object property then applying theProperty Descriptorof the property to the target object using [[DefineOwnProperty]] will not throw an exception.

10.5.7 [[HasProperty]] ( P )

The [[HasProperty]] internal method of aProxy exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"has").
  7. If trap isundefined, then
    1. Return ? target.[[HasProperty]](P).
  8. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target, P »)).
  9. If booleanTrapResult isfalse, then
    1. Let targetDesc be ? target.[[GetOwnProperty]](P).
    2. If targetDesc is notundefined, then
      1. If targetDesc.[[Configurable]] isfalse, throw aTypeErrorexception.
      2. Let extensibleTarget be ? IsExtensible(target).
      3. If extensibleTarget isfalse, throw aTypeErrorexception.
  10. Return booleanTrapResult.
Note

[[HasProperty]] for proxy objects enforces the following invariants:

  • The result of [[HasProperty]] is a Boolean value.
  • A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.

10.5.8 [[Get]] ( P, Receiver )

The [[Get]] internal method of aProxy exotic objectO takes arguments P (a property key) and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"get").
  7. If trap isundefined, then
    1. Return ? target.[[Get]](P, Receiver).
  8. Let trapResult be ? Call(trap, handler, « target, P, Receiver »).
  9. Let targetDesc be ? target.[[GetOwnProperty]](P).
  10. If targetDesc is notundefinedand targetDesc.[[Configurable]] isfalse, then
    1. IfIsDataDescriptor(targetDesc) istrueand targetDesc.[[Writable]] isfalse, then
      1. IfSameValue(trapResult, targetDesc.[[Value]]) isfalse, throw aTypeErrorexception.
    2. IfIsAccessorDescriptor(targetDesc) istrueand targetDesc.[[Get]] isundefined, then
      1. If trapResult is notundefined, throw aTypeErrorexception.
  11. Return trapResult.
Note

[[Get]] for proxy objects enforces the following invariants:

  • The value reported for a property must be the same as the value of the corresponding target object property if the target object property is a non-writable, non-configurable owndata property.
  • The value reported for a property must beundefinedif the corresponding target object property is a non-configurable ownaccessor propertythat hasundefinedas its [[Get]] attribute.

10.5.9 [[Set]] ( P, V, Receiver )

The [[Set]] internal method of aProxy exotic objectO takes arguments P (a property key), V (anECMAScript language value), and Receiver (anECMAScript language value). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"set").
  7. If trap isundefined, then
    1. Return ? target.[[Set]](P, V, Receiver).
  8. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target, P, V, Receiver »)).
  9. If booleanTrapResult isfalse, returnfalse.
  10. Let targetDesc be ? target.[[GetOwnProperty]](P).
  11. If targetDesc is notundefinedand targetDesc.[[Configurable]] isfalse, then
    1. IfIsDataDescriptor(targetDesc) istrueand targetDesc.[[Writable]] isfalse, then
      1. IfSameValue(V, targetDesc.[[Value]]) isfalse, throw aTypeErrorexception.
    2. IfIsAccessorDescriptor(targetDesc) istrue, then
      1. If targetDesc.[[Set]] isundefined, throw aTypeErrorexception.
  12. Returntrue.
Note

[[Set]] for proxy objects enforces the following invariants:

  • The result of [[Set]] is a Boolean value.
  • Cannot change the value of a property to be different from the value of the corresponding target object property if the corresponding target object property is a non-writable, non-configurable owndata property.
  • Cannot set the value of a property if the corresponding target object property is a non-configurable ownaccessor propertythat hasundefinedas its [[Set]] attribute.

10.5.10 [[Delete]] ( P )

The [[Delete]] internal method of aProxy exotic objectO takes argument P (a property key). It performs the following steps when called:

  1. Assert:IsPropertyKey(P) istrue.
  2. Let handler be O.[[ProxyHandler]].
  3. If handler isnull, throw aTypeErrorexception.
  4. Assert:Type(handler) is Object.
  5. Let target be O.[[ProxyTarget]].
  6. Let trap be ? GetMethod(handler,"deleteProperty").
  7. If trap isundefined, then
    1. Return ? target.[[Delete]](P).
  8. Let booleanTrapResult be ! ToBoolean(?Call(trap, handler, « target, P »)).
  9. If booleanTrapResult isfalse, returnfalse.
  10. Let targetDesc be ? target.[[GetOwnProperty]](P).
  11. If targetDesc isundefined, returntrue.
  12. If targetDesc.[[Configurable]] isfalse, throw aTypeErrorexception.
  13. Let extensibleTarget be ? IsExtensible(target).
  14. If extensibleTarget isfalse, throw aTypeErrorexception.
  15. Returntrue.
Note

[[Delete]] for proxy objects enforces the following invariants:

  • The result of [[Delete]] is a Boolean value.
  • A property cannot be reported as deleted, if it exists as a non-configurable own property of the target object.
  • A property cannot be reported as deleted, if it exists as an own property of the target object and the target object is non-extensible.

10.5.11 [[OwnPropertyKeys]] ( )

The [[OwnPropertyKeys]] internal method of aProxy exotic objectO takes no arguments. It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler,"ownKeys").
  6. If trap isundefined, then
    1. Return ? target.[[OwnPropertyKeys]]().
  7. Let trapResultArray be ? Call(trap, handler, « target »).
  8. Let trapResult be ? CreateListFromArrayLike(trapResultArray, « String, Symbol »).
  9. If trapResult contains any duplicate entries, throw aTypeErrorexception.
  10. Let extensibleTarget be ? IsExtensible(target).
  11. Let targetKeys be ? target.[[OwnPropertyKeys]]().
  12. Assert: targetKeys is aListwhose elements are only String and Symbol values.
  13. Assert: targetKeys contains no duplicate entries.
  14. Let targetConfigurableKeys be a new emptyList.
  15. Let targetNonconfigurableKeys be a new emptyList.
  16. For each element key of targetKeys, do
    1. Let desc be ? target.[[GetOwnProperty]](key).
    2. If desc is notundefinedand desc.[[Configurable]] isfalse, then
      1. Append key as an element of targetNonconfigurableKeys.
    3. Else,
      1. Append key as an element of targetConfigurableKeys.
  17. If extensibleTarget istrueand targetNonconfigurableKeys is empty, then
    1. Return trapResult.
  18. Let uncheckedResultKeys be aListwhose elements are the elements of trapResult.
  19. For each element key of targetNonconfigurableKeys, do
    1. If key is not an element of uncheckedResultKeys, throw aTypeErrorexception.
    2. Remove key from uncheckedResultKeys.
  20. If extensibleTarget istrue, return trapResult.
  21. For each element key of targetConfigurableKeys, do
    1. If key is not an element of uncheckedResultKeys, throw aTypeErrorexception.
    2. Remove key from uncheckedResultKeys.
  22. If uncheckedResultKeys is not empty, throw aTypeErrorexception.
  23. Return trapResult.
Note

[[OwnPropertyKeys]] for proxy objects enforces the following invariants:

  • The result of [[OwnPropertyKeys]] is aList.
  • The returnedListcontains no duplicate entries.
  • The Type of each resultListelement is either String or Symbol.
  • The resultListmust contain the keys of all non-configurable own properties of the target object.
  • If the target object is not extensible, then the resultListmust contain all the keys of the own properties of the target object and no other values.

10.5.12 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of aProxy exotic objectO takes arguments thisArgument (anECMAScript language value) and argumentsList (aListof ECMAScript language values). It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler,"apply").
  6. If trap isundefined, then
    1. Return ? Call(target, thisArgument, argumentsList).
  7. Let argArray be ! CreateArrayFromList(argumentsList).
  8. Return ? Call(trap, handler, « target, thisArgument, argArray »).
Note

AProxy exotic objectonly has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.

10.5.13 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of aProxy exotic objectO takes arguments argumentsList (aListof ECMAScript language values) and newTarget (aconstructor). It performs the following steps when called:

  1. Let handler be O.[[ProxyHandler]].
  2. If handler isnull, throw aTypeErrorexception.
  3. Assert:Type(handler) is Object.
  4. Let target be O.[[ProxyTarget]].
  5. Assert:IsConstructor(target) istrue.
  6. Let trap be ? GetMethod(handler,"construct").
  7. If trap isundefined, then
    1. Return ? Construct(target, argumentsList, newTarget).
  8. Let argArray be ! CreateArrayFromList(argumentsList).
  9. Let newObj be ? Call(trap, handler, « target, argArray, newTarget »).
  10. IfType(newObj) is not Object, throw aTypeErrorexception.
  11. Return newObj.
Note 1

AProxy exotic objectonly has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.

Note 2

[[Construct]] for proxy objects enforces the following invariants:

  • The result of [[Construct]] must be an Object.

10.5.14 ProxyCreate ( target, handler )

The abstract operation ProxyCreate takes arguments target and handler. It is used to specify the creation of new Proxy exotic objects. It performs the following steps when called:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. IfType(handler) is not Object, throw aTypeErrorexception.
  3. Let P be ! MakeBasicObject(« [[ProxyHandler]], [[ProxyTarget]] »).
  4. Set P's essential internal methods, except for [[Call]] and [[Construct]], to the definitions specified in10.5.
  5. IfIsCallable(target) istrue, then
    1. Set P.[[Call]] as specified in10.5.12.
    2. IfIsConstructor(target) istrue, then
      1. Set P.[[Construct]] as specified in10.5.13.
  6. Set P.[[ProxyTarget]] to target.
  7. Set P.[[ProxyHandler]] to handler.
  8. Return P.

11 ECMAScript Language: Source Code

11.1 Source Text

Syntax

SourceCharacter::any Unicode code point

ECMAScript code is expressed using Unicode. ECMAScript source text is a sequence of code points. All Unicode code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in source text where permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source text is not relevant to this specification. Regardless of the external source text encoding, a conforming ECMAScript implementation processes the source text as if it was an equivalent sequence ofSourceCharactervalues, eachSourceCharacterbeing a Unicode code point. Conforming ECMAScript implementations are not required to perform any normalization of source text, or behave as though they were performing normalization of source text.

The components of a combining character sequence are treated as individual Unicode code points even though a user might think of the whole sequence as a single character.

Note

In string literals, regular expression literals, template literals and identifiers, any Unicode code point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric value. Within a comment, such an escape sequence is effectively ignored as part of the comment.

ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring within a string literal in an ECMAScript program always contributes to the literal and is never interpreted as a line terminator or as a code point that might terminate the string literal.

11.1.1 Static Semantics: UTF16EncodeCodePoint ( cp )

The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point). It performs the following steps when called:

  1. Assert: 0 ≤ cp ≤ 0x10FFFF.
  2. If cp ≤ 0xFFFF, return the String value consisting of the code unit whose value is cp.
  3. Let cu1 be the code unit whose value isfloor((cp - 0x10000) / 0x400) + 0xD800.
  4. Let cu2 be the code unit whose value is ((cp - 0x10000)modulo0x400) + 0xDC00.
  5. Return thestring-concatenationof cu1 and cu2.

11.1.2 Static Semantics: CodePointsToString ( text )

The abstract operation CodePointsToString takes argument text (a sequence of Unicode code points). It converts text into a String value, as described in6.1.4. It performs the following steps when called:

  1. Let result be the empty String.
  2. For each code point cp of text, do
    1. Set result to thestring-concatenationof result and ! UTF16EncodeCodePoint(cp).
  3. Return result.

11.1.3 Static Semantics: UTF16SurrogatePairToCodePoint ( lead, trail )

The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and trail (a code unit). Two code units that form a UTF-16surrogate pairare converted to a code point. It performs the following steps when called:

  1. Assert: lead is aleading surrogateand trail is atrailing surrogate.
  2. Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) + 0x10000.
  3. Return the code point cp.

11.1.4 Static Semantics: CodePointAt ( string, position )

The abstract operation CodePointAt takes arguments string (a String) and position (a non-negativeinteger). It interprets string as a sequence of UTF-16 encoded code points, as described in6.1.4, and reads from it a single code point starting with the code unit at index position. It performs the following steps when called:

  1. Let size be the length of string.
  2. Assert: position ≥ 0 and position < size.
  3. Let first be the code unit at index position within string.
  4. Let cp be the code point whose numeric value is that of first.
  5. If first is not aleading surrogateortrailing surrogate, then
    1. Return theRecord{ [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:false}.
  6. If first is atrailing surrogateor position + 1 = size, then
    1. Return theRecord{ [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:true}.
  7. Let second be the code unit at index position + 1 within string.
  8. If second is not atrailing surrogate, then
    1. Return theRecord{ [[CodePoint]]: cp, [[CodeUnitCount]]: 1, [[IsUnpairedSurrogate]]:true}.
  9. Set cp to ! UTF16SurrogatePairToCodePoint(first, second).
  10. Return theRecord{ [[CodePoint]]: cp, [[CodeUnitCount]]: 2, [[IsUnpairedSurrogate]]:false}.

11.1.5 Static Semantics: StringToCodePoints ( string )

The abstract operation StringToCodePoints takes argument string (a String). It returns the sequence of Unicode code points that results from interpreting string as UTF-16 encoded Unicode text as described in6.1.4. It performs the following steps when called:

  1. Let codePoints be a new emptyList.
  2. Let size be the length of string.
  3. Let position be 0.
  4. Repeat, while position < size,
    1. Let cp be ! CodePointAt(string, position).
    2. Append cp.[[CodePoint]] to codePoints.
    3. Set position to position + cp.[[CodeUnitCount]].
  5. Return codePoints.

11.1.6 Static Semantics: ParseText ( sourceText, goalSymbol )

The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars). It performs the following steps when called:

  1. Attempt to parse sourceText using goalSymbol as thegoal symbol, and analyse the parse result for anyearly errorconditions. Parsing andearly errordetection may be interleaved in animplementation-definedmanner.
  2. If the parse succeeded and no early errors were found, return theParse Node(an instance of goalSymbol) at the root of the parse tree resulting from the parse.
  3. Otherwise, return aListof one or moreSyntaxErrorobjects representing the parsing errors and/or early errors. If more than one parsing error orearly erroris present, the number and ordering of error objects in the list isimplementation-defined, but at least one must be present.
Note 1

Consider a text that has anearly errorat a particular point, and also a syntax error at a later point. An implementation that does a parse pass followed by an early errors pass might report the syntax error and not proceed to the early errors pass. An implementation that interleaves the two activities might report theearly errorand not proceed to find the syntax error. A third implementation might report both errors. All of these behaviours are conformant.

Note 2

See also clause17.

11.2 Types of Source Code

There are four types of ECMAScript code:

Note 1

Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions (15.3), Method Definitions (15.4), Generator Function Definitions (15.5), Async Function Definitions (15.8), Async Generator Function Definitions (15.6), and Async Arrow Functions (15.9). Function code is also derived from the arguments to the Functionconstructor(20.2.1.1), the GeneratorFunctionconstructor(27.3.1.1), and the AsyncFunctionconstructor(27.7.1.1).

Note 2

The practical effect of including theBindingIdentifierin function code is that the Early Errors forstrict mode codeare applied to aBindingIdentifierthat is the name of a function whose body contains a "use strict" directive, even if the surrounding code is notstrict mode code.

11.2.1 Directive Prologues and the Use Strict Directive

A Directive Prologue is the longest sequence ofExpressionStatements occurring as the initialStatementListItems orModuleItems of aFunctionBody, aScriptBody, or aModuleBodyand where eachExpressionStatementin the sequence consists entirely of aStringLiteraltoken followed by a semicolon. The semicolon may appear explicitly or may be inserted by automatic semicolon insertion (12.9). ADirective Prologuemay be an empty sequence.

A Use Strict Directive is anExpressionStatementin aDirective ProloguewhoseStringLiteralis either of the exact code point sequences "use strict" or 'use strict'. AUse Strict Directivemay not contain anEscapeSequenceorLineContinuation.

ADirective Prologuemay contain more than oneUse Strict Directive. However, an implementation may issue a warning if this occurs.

Note

TheExpressionStatements of aDirective Prologueare evaluated normally during evaluation of the containing production. Implementations may define implementation specific meanings forExpressionStatements which are not aUse Strict Directiveand which occur in aDirective Prologue. If an appropriate notification mechanism exists, an implementation should issue a warning if it encounters in aDirective PrologueanExpressionStatementthat is not aUse Strict Directiveand which does not have a meaning defined by the implementation.

11.2.2 Strict Mode Code

An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics (4.3.2). Code is interpreted as strict mode code in the following situations:

ECMAScript code that is not strict mode code is called non-strict code.

11.2.3 Non-ECMAScript Functions

An ECMAScript implementation may support the evaluation of function exotic objects whose evaluative behaviour is expressed in somehost-definedform of executable code other than via ECMAScript code. Whether afunction objectis an ECMAScript code function or a non-ECMAScript function is not semantically observable from the perspective of an ECMAScript code function that calls or is called by such a non-ECMAScript function.

12 ECMAScript Language: Lexical Grammar

The source text of an ECMAScriptScriptorModuleis first converted into a sequence of input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of code points as the next input element.

There are several situations where the identification of lexical input elements is sensitive to the syntactic grammar context that is consuming the input elements. This requires multiple goal symbols for the lexical grammar. TheInputElementRegExpOrTemplateTailgoal is used in syntactic grammar contexts where aRegularExpressionLiteral, aTemplateMiddle, or aTemplateTailis permitted. TheInputElementRegExpgoal symbolis used in all syntactic grammar contexts where aRegularExpressionLiteralis permitted but neither aTemplateMiddle, nor aTemplateTailis permitted. TheInputElementTemplateTailgoal is used in all syntactic grammar contexts where aTemplateMiddleor aTemplateTailis permitted but aRegularExpressionLiteralis not permitted. In all other contexts,InputElementDivis used as the lexicalgoal symbol.

Note

The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or division-assignment, and a leadingRegularExpressionLiteralare permitted. This is not affected by semicolon insertion (see12.9); in examples such as the following:

a = b
/hi/g.exec(c).map(d);

where the first non-whitespace, non-comment code point after aLineTerminatoris U+002F (SOLIDUS) and the syntactic context allows division or division-assignment, no semicolon is inserted at theLineTerminator. That is, the above example is interpreted in the same way as:

a = b / hi / g.exec(c).map(d);

Syntax

InputElementDiv::WhiteSpaceLineTerminatorCommentCommonTokenDivPunctuatorRightBracePunctuatorInputElementRegExp::WhiteSpaceLineTerminatorCommentCommonTokenRightBracePunctuatorRegularExpressionLiteralInputElementRegExpOrTemplateTail::WhiteSpaceLineTerminatorCommentCommonTokenRegularExpressionLiteralTemplateSubstitutionTailInputElementTemplateTail::WhiteSpaceLineTerminatorCommentCommonTokenDivPunctuatorTemplateSubstitutionTail

12.1 Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of a range of text in the absence of higher-level protocols for this (such as mark-up languages).

It is useful to allow format-control characters in source text to facilitate editing and display. All format control characters may be used within comments, and within string literals, template literals, and regular expression literals.

U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to make necessary distinctions when forming words or phrases in certain languages. In ECMAScript source text these code points may also be used in anIdentifierNameafter the first character.

U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP> code points are treated as white space characters (see12.2).

The special treatment of certain format-control characters outside of comments, string literals, and regular expression literals is summarized inTable 37.

Table 37: Format-Control Code Point Usage
Code PointNameAbbreviationUsage
U+200CZERO WIDTH NON-JOINER<ZWNJ>IdentifierPart
U+200DZERO WIDTH JOINER<ZWJ>IdentifierPart
U+FEFFZERO WIDTH NO-BREAK SPACE<ZWNBSP>WhiteSpace

12.2 White Space

White space code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other, but are otherwise insignificant. White space code points may occur between any two tokens and at the start or end of input. White space code points may occur within aStringLiteral, aRegularExpressionLiteral, aTemplate, or aTemplateSubstitutionTailwhere they are considered significant code points forming part of a literal value. They may also occur within aComment, but cannot appear within any other kind of token.

The ECMAScript white space code points are listed inTable 38.

Table 38: White Space Code Points
Code PointNameAbbreviation
U+0009CHARACTER TABULATION<TAB>
U+000BLINE TABULATION<VT>
U+000CFORM FEED (FF)<FF>
U+0020SPACE<SP>
U+00A0NO-BREAK SPACE<NBSP>
U+FEFFZERO WIDTH NO-BREAK SPACE<ZWNBSP>
Other category “Zs”Any other Unicode “Space_Separator” code point<USP>

ECMAScript implementations must recognize asWhiteSpacecode points listed in the “Space_Separator” (“Zs”) category.

Note

Other than for the code points listed inTable 38, ECMAScriptWhiteSpaceintentionally excludes all code points that have the Unicode “White_Space” property but which are not classified in category “Space_Separator” (“Zs”).

Syntax

WhiteSpace::<TAB><VT><FF><SP><NBSP><ZWNBSP><USP>

12.3 Line Terminators

Like white space code points, line terminator code points are used to improve source text readability and to separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar. Line terminators also affect the process of automatic semicolon insertion (12.9). A line terminator cannot occur within any token except aStringLiteral,Template, orTemplateSubstitutionTail. <LF> and <CR> line terminators cannot occur within aStringLiteraltoken except as part of aLineContinuation.

A line terminator can occur within aMultiLineCommentbut cannot occur within aSingleLineComment.

Line terminators are included in the set of white space code points that are matched by the \s class in regular expressions.

The ECMAScript line terminator code points are listed inTable 39.

Table 39: Line Terminator Code Points
Code PointUnicode NameAbbreviation
U+000ALINE FEED (LF)<LF>
U+000DCARRIAGE RETURN (CR)<CR>
U+2028LINE SEPARATOR<LS>
U+2029PARAGRAPH SEPARATOR<PS>

Only the Unicode code points inTable 39are treated as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators but are treated as white space if they meet the requirements listed inTable 38. The sequence <CR><LF> is commonly used as a line terminator. It should be considered a singleSourceCharacterfor the purpose of reporting line numbers.

Syntax

LineTerminator::<LF><CR><LS><PS>LineTerminatorSequence::<LF><CR>[lookahead ≠<LF>]<LS><PS><CR><LF>

12.4 Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.

Because a single-line comment can contain any Unicode code point except aLineTerminatorcode point, and because of the general rule that a token is always as long as possible, a single-line comment always consists of all code points from the // marker to the end of the line. However, theLineTerminatorat the end of the line is not considered to be part of the single-line comment; it is recognized separately by the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is very important, because it implies that the presence or absence of single-line comments does not affect the process of automatic semicolon insertion (see12.9).

Comments behave like white space and are discarded except that, if aMultiLineCommentcontains a line terminator code point, then the entire comment is considered to be aLineTerminatorfor purposes of parsing by the syntactic grammar.

Syntax

Comment::MultiLineCommentSingleLineCommentMultiLineComment::/*MultiLineCommentCharsopt*/MultiLineCommentChars::MultiLineNotAsteriskCharMultiLineCommentCharsopt*PostAsteriskCommentCharsoptPostAsteriskCommentChars::MultiLineNotForwardSlashOrAsteriskCharMultiLineCommentCharsopt*PostAsteriskCommentCharsoptMultiLineNotAsteriskChar::SourceCharacterbut not*MultiLineNotForwardSlashOrAsteriskChar::SourceCharacterbut not one of/or*SingleLineComment:://SingleLineCommentCharsoptSingleLineCommentChars::SingleLineCommentCharSingleLineCommentCharsoptSingleLineCommentChar::SourceCharacterbut notLineTerminator

A number of productions in this section are given alternative definitions in sectionB.1.3

12.5 Tokens

Syntax

CommonToken::IdentifierNamePrivateIdentifierPunctuatorNumericLiteralStringLiteralTemplateNote

TheDivPunctuator,RegularExpressionLiteral,RightBracePunctuator, andTemplateSubstitutionTailproductions derive additional tokens that are not included in theCommonTokenproduction.

12.6 Names and Keywords

IdentifierNameandReservedWordare tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31, Identifier and Pattern Syntax, with some small modifications.ReservedWordis an enumerated subset ofIdentifierName. The syntactic grammar definesIdentifieras anIdentifierNamethat is not aReservedWord. The Unicode identifier grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the specified categories in the latest version of the Unicode standard must be treated as in those categories by all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points defined in later editions of the Unicode Standard.

Note 1

This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are permitted anywhere in anIdentifierName, and the code points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the first code point of anIdentifierName.

Unicode escape sequences are permitted in anIdentifierName, where they contribute a single Unicode code point to theIdentifierName. The code point is expressed by theCodePointof theUnicodeEscapeSequence(see12.8.4). The \ preceding theUnicodeEscapeSequenceand the u and { } code units, if they appear, do not contribute code points to theIdentifierName. AUnicodeEscapeSequencecannot be used to put a code point into anIdentifierNamethat would otherwise be illegal. In other words, if a \UnicodeEscapeSequencesequence were replaced by theSourceCharacterit contributes, the result must still be a validIdentifierNamethat has the exact same sequence ofSourceCharacterelements as the originalIdentifierName. All interpretations ofIdentifierNamewithin this specification are based upon their actual code points regardless of whether or not an escape sequence was used to contribute any particular code point.

TwoIdentifierNames that are canonically equivalent according to the Unicode standard are not equal unless, after replacement of eachUnicodeEscapeSequence, they are represented by the exact same sequence of code points.

Syntax

PrivateIdentifier::#IdentifierNameIdentifierName::IdentifierStartIdentifierNameIdentifierPartIdentifierStart::UnicodeIDStart$_\UnicodeEscapeSequenceIdentifierPart::UnicodeIDContinue$\UnicodeEscapeSequence<ZWNJ><ZWJ>UnicodeIDStart::any Unicode code point with the Unicode property “ID_Start”UnicodeIDContinue::any Unicode code point with the Unicode property “ID_Continue”

The definitions of the nonterminalUnicodeEscapeSequenceis given in12.8.4.

Note 2

The nonterminalIdentifierPartderives _ viaUnicodeIDContinue.

Note 3

The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.

12.6.1 Identifier Names

12.6.1.1 Static Semantics: Early Errors

IdentifierStart::\UnicodeEscapeSequenceIdentifierPart::\UnicodeEscapeSequence

12.6.2 Keywords and Reserved Words

A keyword is a token that matchesIdentifierName, but also has a syntactic use; that is, it appears literally, in a fixed width font, in some syntactic production. The keywords of ECMAScript include if, while, async, await, and many others.

A reserved word is anIdentifierNamethat cannot be used as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain contexts. if and while are reserved words. await is reserved only inside async functions and modules. async is not reserved; it can be used as a variable name or statement label without restriction.

This specification uses a combination of grammatical productions andearly errorrules to specify which names are valid identifiers and which are reserved words. All tokens in theReservedWordlist below, except for await and yield, are unconditionally reserved. Exceptions for await and yield are specified in13.1, using parameterized syntactic productions. Lastly, severalearly errorrules restrict the set of valid identifiers. See13.1.1,14.3.1.1,14.7.5.1, and15.7.1. In summary, there are five categories of identifier names:

  • Those that are always allowed as identifiers, and are not keywords, such as Math, window, toString, and _;

  • Those that are never allowed as identifiers, namely theReservedWords listed below except await and yield;

  • Those that are contextually allowed as identifiers, namely await and yield;

  • Those that are contextually disallowed as identifiers, instrict mode code: let, static, implements, interface, package, private, protected, and public;

  • Those that are always allowed as identifiers, but also appear as keywords within certain syntactic productions, at places whereIdentifieris not allowed: as, async, from, get, meta, of, set, and target.

The term conditional keyword, or contextual keyword, is sometimes used to refer to the keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as keywords in others.

Syntax

ReservedWord::one ofawaitbreakcasecatchclassconstcontinuedebuggerdefaultdeletedoelseenumexportextendsfalsefinallyforfunctionifimportininstanceofnewnullreturnsuperswitchthisthrowtruetrytypeofvarvoidwhilewithyieldNote 1

Per5.1.5, keywords in the grammar match literal sequences of specificSourceCharacterelements. A code point in a keyword cannot be expressed by a \UnicodeEscapeSequence.

AnIdentifierNamecan contain \UnicodeEscapeSequences, but it is not possible to declare a variable named "else" by spelling it els\u{65}. Theearly errorrules in13.1.1rule out identifiers with the sameStringValueas a reserved word.

Note 2

enum is not currently used as a keyword in this specification. It is a future reserved word, set aside for use as a keyword in future language extensions.

Similarly, implements, interface, package, private, protected, and public are future reserved words instrict mode code.

Note 3

The names arguments and eval are not keywords, but they are subject to some restrictions instrict mode code. See13.1.1,8.5.4,15.2.1,15.5.1,15.6.1, and15.8.1.

12.7 Punctuators

Syntax

Punctuator::OptionalChainingPunctuatorOtherPunctuatorOptionalChainingPunctuator::?.[lookahead ∉DecimalDigit]OtherPunctuator::one of{()[]....;,<><=>===!====!==+-*%**++--<<>>>>>&|^!~&&||???:=+=-=*=%=**=<<=>>=>>>=&=|=^=&&=||=??==>DivPunctuator:://=RightBracePunctuator::}

12.8 Literals

12.8.1 Null Literals

Syntax

NullLiteral::null

12.8.2 Boolean Literals

Syntax

BooleanLiteral::truefalse

12.8.3 Numeric Literals

Syntax

NumericLiteralSeparator::_NumericLiteral::DecimalLiteralDecimalBigIntegerLiteralNonDecimalIntegerLiteral[+Sep]NonDecimalIntegerLiteral[+Sep]BigIntLiteralSuffixDecimalBigIntegerLiteral::0BigIntLiteralSuffixNonZeroDigitDecimalDigits[+Sep]optBigIntLiteralSuffixNonZeroDigitNumericLiteralSeparatorDecimalDigits[+Sep]BigIntLiteralSuffixNonDecimalIntegerLiteral[Sep]::BinaryIntegerLiteral[?Sep]OctalIntegerLiteral[?Sep]HexIntegerLiteral[?Sep]BigIntLiteralSuffix::nDecimalLiteral::DecimalIntegerLiteral.DecimalDigits[+Sep]optExponentPart[+Sep]opt.DecimalDigits[+Sep]ExponentPart[+Sep]optDecimalIntegerLiteralExponentPart[+Sep]optDecimalIntegerLiteral::0NonZeroDigitNonZeroDigitNumericLiteralSeparatoroptDecimalDigits[+Sep]DecimalDigits[Sep]::DecimalDigitDecimalDigits[?Sep]DecimalDigit[+Sep]DecimalDigits[+Sep]NumericLiteralSeparatorDecimalDigitDecimalDigit::one of0123456789NonZeroDigit::one of123456789ExponentPart[Sep]::ExponentIndicatorSignedInteger[?Sep]ExponentIndicator::one ofeESignedInteger[Sep]::DecimalDigits[?Sep]+DecimalDigits[?Sep]-DecimalDigits[?Sep]BinaryIntegerLiteral[Sep]::0bBinaryDigits[?Sep]0BBinaryDigits[?Sep]BinaryDigits[Sep]::BinaryDigitBinaryDigits[?Sep]BinaryDigit[+Sep]BinaryDigits[+Sep]NumericLiteralSeparatorBinaryDigitBinaryDigit::one of01OctalIntegerLiteral[Sep]::0oOctalDigits[?Sep]0OOctalDigits[?Sep]OctalDigits[Sep]::OctalDigitOctalDigits[?Sep]OctalDigit[+Sep]OctalDigits[+Sep]NumericLiteralSeparatorOctalDigitOctalDigit::one of01234567HexIntegerLiteral[Sep]::0xHexDigits[?Sep]0XHexDigits[?Sep]HexDigits[Sep]::HexDigitHexDigits[?Sep]HexDigit[+Sep]HexDigits[+Sep]NumericLiteralSeparatorHexDigitHexDigit::one of0123456789abcdefABCDEF

TheSourceCharacterimmediately following aNumericLiteralmust not be anIdentifierStartorDecimalDigit.

Note

For example: 3in is an error and not the two input elements 3 and in.

A conforming implementation, when processingstrict mode code, must not extend, as described inB.1.1, the syntax ofNumericLiteralto includeLegacyOctalIntegerLiteral, nor extend the syntax ofDecimalIntegerLiteralto includeNonOctalDecimalIntegerLiteral.

12.8.3.1 Static Semantics: MV

A numeric literal stands for a value of the Number type or the BigInt type.

12.8.3.2 Static Semantics: NumericValue

NumericLiteral::DecimalLiteral
  1. ReturnRoundMVResult(MV ofDecimalLiteral).
NumericLiteral::NonDecimalIntegerLiteral
  1. Return𝔽(MV ofNonDecimalIntegerLiteral).
NumericLiteral::NonDecimalIntegerLiteralBigIntLiteralSuffix
  1. Return the BigInt value that represents the MV ofNonDecimalIntegerLiteral.
DecimalBigIntegerLiteral::0BigIntLiteralSuffix
  1. Return0.
DecimalBigIntegerLiteral::NonZeroDigitBigIntLiteralSuffix
  1. Return the BigInt value that represents the MV ofNonZeroDigit.
DecimalBigIntegerLiteral::NonZeroDigitDecimalDigitsBigIntLiteralSuffixNonZeroDigitNumericLiteralSeparatorDecimalDigitsBigIntLiteralSuffix
  1. Let n be the number of code points inDecimalDigits, excluding all occurrences ofNumericLiteralSeparator.
  2. Let mv be (the MV ofNonZeroDigit× 10n) plus the MV ofDecimalDigits.
  3. Return(mv).

12.8.4 String Literals

Note 1

A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code points may also be represented by an escape sequence. All code points may appear literally in a string literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16 encoded as defined in11.1.1. Code points belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All other code points are encoded as two code unit elements of the string.

Syntax

StringLiteral::"DoubleStringCharactersopt"'SingleStringCharactersopt'DoubleStringCharacters::DoubleStringCharacterDoubleStringCharactersoptSingleStringCharacters::SingleStringCharacterSingleStringCharactersoptDoubleStringCharacter::SourceCharacterbut not one of"or\orLineTerminator<LS><PS>\EscapeSequenceLineContinuationSingleStringCharacter::SourceCharacterbut not one of'or\orLineTerminator<LS><PS>\EscapeSequenceLineContinuationLineContinuation::\LineTerminatorSequenceEscapeSequence::CharacterEscapeSequence0[lookahead ∉DecimalDigit]HexEscapeSequenceUnicodeEscapeSequence

A conforming implementation, when processingstrict mode code, must not extend the syntax ofEscapeSequenceto includeLegacyOctalEscapeSequenceorNonOctalDecimalEscapeSequenceas described inB.1.2.

CharacterEscapeSequence::SingleEscapeCharacterNonEscapeCharacterSingleEscapeCharacter::one of'"\bfnrtvNonEscapeCharacter::SourceCharacterbut not one ofEscapeCharacterorLineTerminatorEscapeCharacter::SingleEscapeCharacterDecimalDigitxuHexEscapeSequence::xHexDigitHexDigitUnicodeEscapeSequence::uHex4Digitsu{CodePoint}Hex4Digits::HexDigitHexDigitHexDigitHexDigit

The definition of the nonterminalHexDigitis given in12.8.3.SourceCharacteris defined in11.1.

Note 2

<LF> and <CR> cannot appear in a string literal, except as part of aLineContinuationto produce the empty code points sequence. The proper way to include either in the String value of a string literal is to use an escape sequence such as \n or \u000A.

12.8.4.1 Static Semantics: SV

A string literal stands for a value of the String type. SV produces String values for string literals through recursive application on the various parts of the string literal. As part of this process, some Unicode code points within the string literal are interpreted as having amathematical value, as described below or in12.8.3.

Table 40: String Single Character Escape Sequences
Escape SequenceCode Unit ValueUnicode Character NameSymbol
\b0x0008BACKSPACE<BS>
\t0x0009CHARACTER TABULATION<HT>
\n0x000ALINE FEED (LF)<LF>
\v0x000BLINE TABULATION<VT>
\f0x000CFORM FEED (FF)<FF>
\r0x000DCARRIAGE RETURN (CR)<CR>
\"0x0022QUOTATION MARK"
\'0x0027APOSTROPHE'
\\0x005CREVERSE SOLIDUS\

12.8.4.2 Static Semantics: MV

12.8.5 Regular Expression Literals

Note 1

A regular expression literal is an input element that is converted to a RegExp object (see22.2) each time the literal is evaluated. Two regular expression literals in a program evaluate to regular expression objects that never compare as === to each other even if the two literals' contents are identical. A RegExp object may also be created at runtime by new RegExp or calling the RegExpconstructoras a function (see22.2.3).

The productions below describe the syntax for a regular expression literal and are used by the input element scanner to find the end of the regular expression literal. The source text comprising theRegularExpressionBodyand theRegularExpressionFlagsare subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).

An implementation may extend the ECMAScript Regular Expression grammar defined in22.2.1, but it must not extend theRegularExpressionBodyandRegularExpressionFlagsproductions defined below or the productions used by these productions.

Syntax

RegularExpressionLiteral::/RegularExpressionBody/RegularExpressionFlagsRegularExpressionBody::RegularExpressionFirstCharRegularExpressionCharsRegularExpressionChars::[empty]RegularExpressionCharsRegularExpressionCharRegularExpressionFirstChar::RegularExpressionNonTerminatorbut not one of*or\or/or[RegularExpressionBackslashSequenceRegularExpressionClassRegularExpressionChar::RegularExpressionNonTerminatorbut not one of\or/or[RegularExpressionBackslashSequenceRegularExpressionClassRegularExpressionBackslashSequence::\RegularExpressionNonTerminatorRegularExpressionNonTerminator::SourceCharacterbut notLineTerminatorRegularExpressionClass::[RegularExpressionClassChars]RegularExpressionClassChars::[empty]RegularExpressionClassCharsRegularExpressionClassCharRegularExpressionClassChar::RegularExpressionNonTerminatorbut not one of]or\RegularExpressionBackslashSequenceRegularExpressionFlags::[empty]RegularExpressionFlagsIdentifierPartNote 2

Regular expression literals may not be empty; instead of representing an empty regular expression literal, the code unit sequence // starts a single-line comment. To specify an empty regular expression, use: /(?:)/.

12.8.5.1 Static Semantics: Early Errors

RegularExpressionFlags::RegularExpressionFlagsIdentifierPart
  • It is a Syntax Error ifIdentifierPartcontains a Unicode escape sequence.

12.8.5.2 Static Semantics: BodyText

RegularExpressionLiteral::/RegularExpressionBody/RegularExpressionFlags
  1. Return the source text that was recognized asRegularExpressionBody.

12.8.5.3 Static Semantics: FlagText

RegularExpressionLiteral::/RegularExpressionBody/RegularExpressionFlags
  1. Return the source text that was recognized asRegularExpressionFlags.

12.8.6 Template Literal Lexical Components

Syntax

Template::NoSubstitutionTemplateTemplateHeadNoSubstitutionTemplate::`TemplateCharactersopt`TemplateHead::`TemplateCharactersopt${TemplateSubstitutionTail::TemplateMiddleTemplateTailTemplateMiddle::}TemplateCharactersopt${TemplateTail::}TemplateCharactersopt`TemplateCharacters::TemplateCharacterTemplateCharactersoptTemplateCharacter::$[lookahead ≠{]\EscapeSequence\NotEscapeSequenceLineContinuationLineTerminatorSequenceSourceCharacterbut not one of`or\or$orLineTerminatorNotEscapeSequence::0DecimalDigitDecimalDigitbut not0x[lookahead ∉HexDigit]xHexDigit[lookahead ∉HexDigit]u[lookahead ∉HexDigit][lookahead ≠{]uHexDigit[lookahead ∉HexDigit]uHexDigitHexDigit[lookahead ∉HexDigit]uHexDigitHexDigitHexDigit[lookahead ∉HexDigit]u{[lookahead ∉HexDigit]u{NotCodePoint[lookahead ∉HexDigit]u{CodePoint[lookahead ∉HexDigit][lookahead ≠}]NotCodePoint::HexDigits[~Sep]but only if MV ofHexDigits> 0x10FFFFCodePoint::HexDigits[~Sep]but only if MV ofHexDigits≤ 0x10FFFF

A conforming implementation must not use the extended definition ofEscapeSequencedescribed inB.1.2when parsing aTemplateCharacter.

Note

TemplateSubstitutionTailis used by theInputElementTemplateTailalternative lexical goal.

12.8.6.1 Static Semantics: TV

A template literal component is interpreted by TV as a value of the String type. TV is used to construct the indexed components of a template object (colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code point represented by the escape sequence.

12.8.6.2 Static Semantics: TRV

A template literal component is interpreted by TRV as a value of the String type. TRV is used to construct the raw components of a template object (colloquially, the template raw values). TRV is similar toTVwith the difference being that in TRV, escape sequences are interpreted as they appear in the literal.

Note

TVexcludes the code units ofLineContinuationwhile TRV includes them. <CR><LF> and <CR>LineTerminatorSequences are normalized to <LF> for bothTVand TRV. An explicitEscapeSequenceis needed to include a <CR> or <CR><LF> sequence.

12.9 Automatic Semicolon Insertion

Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.

12.9.1 Rules of Automatic Semicolon Insertion

In the following rules, “token” means the actual recognized lexical token determined using the current lexicalgoal symbolas described in clause12.

There are three basic rules of semicolon insertion:

  1. When, as the source text is parsed from left to right, a token (called the offending token) is encountered that is not allowed by any production of the grammar, then a semicolon is automatically inserted before the offending token if one or more of the following conditions is true:

    • The offending token is separated from the previous token by at least oneLineTerminator.
    • The offending token is }.
    • The previous token is ) and the inserted semicolon would then be parsed as the terminating semicolon of a do-while statement (14.7.2).
  2. When, as the source text is parsed from left to right, the end of the input stream of tokens is encountered and the parser is unable to parse the input token stream as a single instance of the goal nonterminal, then a semicolon is automatically inserted at the end of the input stream.
  3. When, as the source text is parsed from left to right, a token is encountered that is allowed by some production of the grammar, but the production is a restricted production and the token would be the first token for a terminal or nonterminal immediately following the annotation “[noLineTerminatorhere]” within the restricted production (and therefore such a token is called a restricted token), and the restricted token is separated from the previous token by at least oneLineTerminator, then a semicolon is automatically inserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become one of the two semicolons in the header of a for statement (see14.7.4).

Note

The following are the only restricted productions in the grammar:

UpdateExpression[Yield, Await]:LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]++LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]--ContinueStatement[Yield, Await]:continue;continue[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];BreakStatement[Yield, Await]:break;break[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];ReturnStatement[Yield, Await]:return;return[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];ThrowStatement[Yield, Await]:throw[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];ArrowFunction[In, Yield, Await]:ArrowParameters[?Yield, ?Await][noLineTerminatorhere]=>ConciseBody[?In]YieldExpression[In, Await]:yieldyield[noLineTerminatorhere]AssignmentExpression[?In, +Yield, ?Await]yield[noLineTerminatorhere]*AssignmentExpression[?In, +Yield, ?Await]

The practical effect of these restricted productions is as follows:

  • When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least oneLineTerminatoroccurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted before the ++ or -- token.
  • When a continue, break, return, throw, or yield token is encountered and aLineTerminatoris encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

The resulting practical advice to ECMAScript programmers is:

  • A postfix ++ or -- operator should appear on the same line as its operand.
  • AnExpressionin a return or throw statement or anAssignmentExpressionin a yield expression should start on the same line as the return, throw, or yield token.
  • ALabelIdentifierin a break or continue statement should be on the same line as the break or continue token.

12.9.2 Examples of Automatic Semicolon Insertion

This section is non-normative.

The source

{ 1 2 } 3

is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In contrast, the source

{ 1
2 } 3

is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the following:

{ 1
;2 ;} 3;

which is a valid ECMAScript sentence.

The source

for (a; b
)

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the semicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of the two semicolons in the header of a for statement.

The source

return
a + b

is transformed by automatic semicolon insertion into the following:

return;
a + b;
Note 1

The expression a + b is not treated as a value to be returned by the return statement, because aLineTerminatorseparates it from the token return.

The source

a = b
++c

is transformed by automatic semicolon insertion into the following:

a = b;
++c;
Note 2

The token ++ is not treated as a postfix operator applying to the variable b, because aLineTerminatoroccurs between b and ++.

The source

if (a > b)
else c = d

is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token, even though no production of the grammar applies at that point, because an automatically inserted semicolon would then be parsed as an empty statement.

The source

a = b + c
(d + e).print()

is not transformed by automatic semicolon insertion, because the parenthesized expression that begins the second line can be interpreted as an argument list for a function call:

a = b + c(d + e).print()

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on automatic semicolon insertion.

12.9.3 Interesting Cases of Automatic Semicolon Insertion

This section is non-normative.

ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon insertion can depend on multiple tokens across line terminators.

As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.

For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The rest of this section describes a number of interesting cases of automatic semicolon insertion in this version of ECMAScript.

12.9.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement Lists

In aStatementList, manyStatementListItems end in semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above, at the end of a line ending an expression, a semicolon is required if the following line begins with any of the following:

  • An opening parenthesis ((). Without a semicolon, the two lines together are treated as aCallExpression.
  • An opening square bracket ([). Without a semicolon, the two lines together are treated as property access, rather than anArrayLiteralorArrayAssignmentPattern.
  • A template literal (`). Without a semicolon, the two lines together are interpreted as a tagged Template (13.3.11), with the previous expression as theMemberExpression.
  • Unary + or -. Without a semicolon, the two lines together are interpreted as a usage of the corresponding binary operator.
  • A RegExp literal. Without a semicolon, the two lines together may be parsed instead as the /MultiplicativeOperator, for example if the RegExp has flags.

12.9.3.2 Cases of Automatic Semicolon Insertion and “[noLineTerminatorhere]”

This section is non-normative.

ECMAScript contains grammar productions which include “[noLineTerminatorhere]”. These productions are sometimes a means to have optional operands in the grammar. Introducing aLineTerminatorin these locations would change the grammar production of a source text by using the grammar production without the optional operand.

The rest of this section describes a number of productions using “[noLineTerminatorhere]” in this version of ECMAScript.

12.9.3.2.1 List of Grammar Productions with Optional Operands and “[noLineTerminatorhere]”

13 ECMAScript Language: Expressions

13.1 Identifiers

Syntax

IdentifierReference[Yield, Await]:Identifier[~Yield]yield[~Await]awaitBindingIdentifier[Yield, Await]:IdentifieryieldawaitLabelIdentifier[Yield, Await]:Identifier[~Yield]yield[~Await]awaitIdentifier:IdentifierNamebut notReservedWordNote

yield and await are permitted asBindingIdentifierin the grammar, and prohibited withstatic semanticsbelow, to prohibit automatic semicolon insertion in cases such as

let
await 0;

13.1.1 Static Semantics: Early Errors

BindingIdentifier:IdentifierIdentifierReference:yieldBindingIdentifier:yieldLabelIdentifier:yield
  • It is a Syntax Error if the code matched by this production is contained instrict mode code.
IdentifierReference:awaitBindingIdentifier:awaitLabelIdentifier:awaitBindingIdentifier[Yield, Await]:yield
  • It is a Syntax Error if this production has a [Yield] parameter.
BindingIdentifier[Yield, Await]:await
  • It is a Syntax Error if this production has an [Await] parameter.
IdentifierReference[Yield, Await]:IdentifierBindingIdentifier[Yield, Await]:IdentifierLabelIdentifier[Yield, Await]:IdentifierIdentifier:IdentifierNamebut notReservedWordNote

StringValueofIdentifierNamenormalizes any Unicode escape sequences inIdentifierNamehence such escapes cannot be used to write anIdentifierwhose code point sequence is the same as aReservedWord.

13.1.2 Static Semantics: StringValue

IdentifierName::IdentifierStartIdentifierNameIdentifierPart
  1. Let idText be the source text matched byIdentifierName.
  2. Let idTextUnescaped be the result of replacing any occurrences of \UnicodeEscapeSequencein idText with the code point represented by theUnicodeEscapeSequence.
  3. Return ! CodePointsToString(idTextUnescaped).
IdentifierReference:yieldBindingIdentifier:yieldLabelIdentifier:yield
  1. Return"yield".
IdentifierReference:awaitBindingIdentifier:awaitLabelIdentifier:await
  1. Return"await".
Identifier:IdentifierNamebut notReservedWord
  1. Return theStringValueofIdentifierName.
PrivateIdentifier::#IdentifierName
  1. Return thestring-concatenationof 0x0023 (NUMBER SIGN) and theStringValueofIdentifierName.

13.1.3 Runtime Semantics: Evaluation

IdentifierReference:Identifier
  1. Return ? ResolveBinding(StringValueofIdentifier).
IdentifierReference:yield
  1. Return ? ResolveBinding("yield").
IdentifierReference:await
  1. Return ? ResolveBinding("await").
Note 1

The result of evaluating anIdentifierReferenceis always a value of type Reference.

Note 2

Innon-strict code, thekeywordyield may be used as an identifier. Evaluating theIdentifierReferenceresolves the binding of yield as if it was anIdentifier. Early Error restriction ensures that such an evaluation only can occur fornon-strict code.

13.2 Primary Expression

Syntax

PrimaryExpression[Yield, Await]:thisIdentifierReference[?Yield, ?Await]LiteralArrayLiteral[?Yield, ?Await]ObjectLiteral[?Yield, ?Await]FunctionExpressionClassExpression[?Yield, ?Await]GeneratorExpressionAsyncFunctionExpressionAsyncGeneratorExpressionRegularExpressionLiteralTemplateLiteral[?Yield, ?Await, ~Tagged]CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]CoverParenthesizedExpressionAndArrowParameterList[Yield, Await]:(Expression[+In, ?Yield, ?Await])(Expression[+In, ?Yield, ?Await],)()(...BindingIdentifier[?Yield, ?Await])(...BindingPattern[?Yield, ?Await])(Expression[+In, ?Yield, ?Await],...BindingIdentifier[?Yield, ?Await])(Expression[+In, ?Yield, ?Await],...BindingPattern[?Yield, ?Await])

Supplemental Syntax

When processing an instance of the production
PrimaryExpression[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation ofCoverParenthesizedExpressionAndArrowParameterListis refined using the following grammar:

ParenthesizedExpression[Yield, Await]:(Expression[+In, ?Yield, ?Await])

13.2.1 The this Keyword

13.2.1.1 Runtime Semantics: Evaluation

PrimaryExpression:this
  1. Return ? ResolveThisBinding().

13.2.2 Identifier Reference

See13.1forIdentifierReference.

13.2.3 Literals

Syntax

Literal:NullLiteralBooleanLiteralNumericLiteralStringLiteral

13.2.3.1 Runtime Semantics: Evaluation

Literal:NullLiteral
  1. Returnnull.
Literal:BooleanLiteral
  1. IfBooleanLiteralis the token false, returnfalse.
  2. IfBooleanLiteralis the token true, returntrue.
Literal:NumericLiteral
  1. Return theNumericValueofNumericLiteralas defined in12.8.3.
Literal:StringLiteral
  1. Return theSVofStringLiteralas defined in12.8.4.1.

13.2.4 Array Initializer

Note

AnArrayLiteralis an expression describing the initialization of an Array object, using a list, of zero or more expressions each of which represents an array element, enclosed in square brackets. The elements need not be literals; they are evaluated each time the array initializer is evaluated.

Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the element list is not preceded by anAssignmentExpression(i.e., a comma at the beginning or after another comma), the missing array element contributes to the length of the Array and increases the index of subsequent elements. Elided array elements are not defined. If an element is elided at the end of an array, that element does not contribute to the length of the Array.

Syntax

ArrayLiteral[Yield, Await]:[Elisionopt][ElementList[?Yield, ?Await]][ElementList[?Yield, ?Await],Elisionopt]ElementList[Yield, Await]:ElisionoptAssignmentExpression[+In, ?Yield, ?Await]ElisionoptSpreadElement[?Yield, ?Await]ElementList[?Yield, ?Await],ElisionoptAssignmentExpression[+In, ?Yield, ?Await]ElementList[?Yield, ?Await],ElisionoptSpreadElement[?Yield, ?Await]Elision:,Elision,SpreadElement[Yield, Await]:...AssignmentExpression[+In, ?Yield, ?Await]

13.2.4.1 Runtime Semantics: ArrayAccumulation

With parameters array and nextIndex.

Elision:,
  1. Let len be nextIndex + 1.
  2. Perform ? Set(array,"length",𝔽(len),true).
  3. NOTE: The above Set throws if len exceeds 232-1.
  4. Return len.
Elision:Elision,
  1. Return the result of performingArrayAccumulationforElisionwith arguments array and nextIndex + 1.
ElementList:ElisionoptAssignmentExpression
  1. IfElisionis present, then
    1. Set nextIndex to the result of performingArrayAccumulationforElisionwith arguments array and nextIndex.
    2. ReturnIfAbrupt(nextIndex).
  2. Let initResult be the result of evaluatingAssignmentExpression.
  3. Let initValue be ? GetValue(initResult).
  4. Let created be ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
  5. Return nextIndex + 1.
ElementList:ElisionoptSpreadElement
  1. IfElisionis present, then
    1. Set nextIndex to the result of performingArrayAccumulationforElisionwith arguments array and nextIndex.
    2. ReturnIfAbrupt(nextIndex).
  2. Return the result of performingArrayAccumulationforSpreadElementwith arguments array and nextIndex.
ElementList:ElementList,ElisionoptAssignmentExpression
  1. Set nextIndex to the result of performingArrayAccumulationforElementListwith arguments array and nextIndex.
  2. ReturnIfAbrupt(nextIndex).
  3. IfElisionis present, then
    1. Set nextIndex to the result of performingArrayAccumulationforElisionwith arguments array and nextIndex.
    2. ReturnIfAbrupt(nextIndex).
  4. Let initResult be the result of evaluatingAssignmentExpression.
  5. Let initValue be ? GetValue(initResult).
  6. Let created be ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), initValue).
  7. Return nextIndex + 1.
ElementList:ElementList,ElisionoptSpreadElement
  1. Set nextIndex to the result of performingArrayAccumulationforElementListwith arguments array and nextIndex.
  2. ReturnIfAbrupt(nextIndex).
  3. IfElisionis present, then
    1. Set nextIndex to the result of performingArrayAccumulationforElisionwith arguments array and nextIndex.
    2. ReturnIfAbrupt(nextIndex).
  4. Return the result of performingArrayAccumulationforSpreadElementwith arguments array and nextIndex.
SpreadElement:...AssignmentExpression
  1. Let spreadRef be the result of evaluatingAssignmentExpression.
  2. Let spreadObj be ? GetValue(spreadRef).
  3. Let iteratorRecord be ? GetIterator(spreadObj).
  4. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return nextIndex.
    3. Let nextValue be ? IteratorValue(next).
    4. Perform ! CreateDataPropertyOrThrow(array, ! ToString(𝔽(nextIndex)), nextValue).
    5. Set nextIndex to nextIndex + 1.
Note

CreateDataPropertyOrThrowis used to ensure that own properties are defined for the array even if the standard built-inArray prototype objecthas been modified in a manner that would preclude the creation of new own properties using [[Set]].

13.2.4.2 Runtime Semantics: Evaluation

ArrayLiteral:[Elisionopt]
  1. Let array be ! ArrayCreate(0).
  2. IfElisionis present, then
    1. Let len be the result of performingArrayAccumulationforElisionwith arguments array and 0.
    2. ReturnIfAbrupt(len).
  3. Return array.
ArrayLiteral:[ElementList]
  1. Let array be ! ArrayCreate(0).
  2. Let len be the result of performingArrayAccumulationforElementListwith arguments array and 0.
  3. ReturnIfAbrupt(len).
  4. Return array.
ArrayLiteral:[ElementList,Elisionopt]
  1. Let array be ! ArrayCreate(0).
  2. Let nextIndex be the result of performingArrayAccumulationforElementListwith arguments array and 0.
  3. ReturnIfAbrupt(nextIndex).
  4. IfElisionis present, then
    1. Let len be the result of performingArrayAccumulationforElisionwith arguments array and nextIndex.
    2. ReturnIfAbrupt(len).
  5. Return array.

13.2.5 Object Initializer

Note 1

An object initializer is an expression describing the initialization of an Object, written in a form resembling a literal. It is a list of zero or more pairs of property keys and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated each time the object initializer is evaluated.

Syntax

ObjectLiteral[Yield, Await]:{}{PropertyDefinitionList[?Yield, ?Await]}{PropertyDefinitionList[?Yield, ?Await],}PropertyDefinitionList[Yield, Await]:PropertyDefinition[?Yield, ?Await]PropertyDefinitionList[?Yield, ?Await],PropertyDefinition[?Yield, ?Await]PropertyDefinition[Yield, Await]:IdentifierReference[?Yield, ?Await]CoverInitializedName[?Yield, ?Await]PropertyName[?Yield, ?Await]:AssignmentExpression[+In, ?Yield, ?Await]MethodDefinition[?Yield, ?Await]...AssignmentExpression[+In, ?Yield, ?Await]PropertyName[Yield, Await]:LiteralPropertyNameComputedPropertyName[?Yield, ?Await]LiteralPropertyName:IdentifierNameStringLiteralNumericLiteralComputedPropertyName[Yield, Await]:[AssignmentExpression[+In, ?Yield, ?Await]]CoverInitializedName[Yield, Await]:IdentifierReference[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]Initializer[In, Yield, Await]:=AssignmentExpression[?In, ?Yield, ?Await]Note 2

MethodDefinitionis defined in15.4.

Note 3

In certain contexts,ObjectLiteralis used as a cover grammar for a more restricted secondary grammar. TheCoverInitializedNameproduction is necessary to fully cover these secondary grammars. However, use of this production results in an early Syntax Error in normal contexts where an actualObjectLiteralis expected.

13.2.5.1 Static Semantics: Early Errors

PropertyDefinition:MethodDefinition

In addition to describing an actual object initializer theObjectLiteralproductions are also used as a cover grammar forObjectAssignmentPatternand may be recognized as part of aCoverParenthesizedExpressionAndArrowParameterList. WhenObjectLiteralappears in a context whereObjectAssignmentPatternis required the following Early Error rules are not applied. In addition, they are not applied when initially parsing aCoverParenthesizedExpressionAndArrowParameterListorCoverCallExpressionAndAsyncArrowHead.

PropertyDefinition:CoverInitializedName
  • Always throw a Syntax Error if code matches this production.
Note 1

This production exists so thatObjectLiteralcan serve as a cover grammar forObjectAssignmentPattern. It cannot occur in an actual object initializer.

ObjectLiteral:{PropertyDefinitionList}ObjectLiteral:{PropertyDefinitionList,}Note 2

TheListreturned byPropertyNameListdoes not include property names defined using aComputedPropertyName.

13.2.5.2 Static Semantics: IsComputedPropertyKey

PropertyName:LiteralPropertyName
  1. Returnfalse.
PropertyName:ComputedPropertyName
  1. Returntrue.

13.2.5.3 Static Semantics: PropertyNameList

PropertyDefinitionList:PropertyDefinition
  1. Let propName bePropNameofPropertyDefinition.
  2. If propName isempty, return a new emptyList.
  3. Return aListwhose sole element is propName.
PropertyDefinitionList:PropertyDefinitionList,PropertyDefinition
  1. Let list bePropertyNameListofPropertyDefinitionList.
  2. Let propName bePropNameofPropertyDefinition.
  3. If propName isempty, return list.
  4. Return thelist-concatenationof list and « propName ».

13.2.5.4 Runtime Semantics: Evaluation

ObjectLiteral:{}
  1. Return ! OrdinaryObjectCreate(%Object.prototype%).
ObjectLiteral:{PropertyDefinitionList}{PropertyDefinitionList,}
  1. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  2. Perform ?PropertyDefinitionEvaluationofPropertyDefinitionListwith argument obj.
  3. Return obj.
LiteralPropertyName:IdentifierName
  1. ReturnStringValueofIdentifierName.
LiteralPropertyName:StringLiteral
  1. Return theSVofStringLiteral.
LiteralPropertyName:NumericLiteral
  1. Let nbr be theNumericValueofNumericLiteral.
  2. Return ! ToString(nbr).
ComputedPropertyName:[AssignmentExpression]
  1. Let exprValue be the result of evaluatingAssignmentExpression.
  2. Let propName be ? GetValue(exprValue).
  3. Return ? ToPropertyKey(propName).

13.2.5.5 Runtime Semantics: PropertyDefinitionEvaluation

With parameter object.

PropertyDefinitionList:PropertyDefinitionList,PropertyDefinition
  1. Perform ?PropertyDefinitionEvaluationofPropertyDefinitionListwith argument object.
  2. Return the result of performingPropertyDefinitionEvaluationofPropertyDefinitionwith argument object.
PropertyDefinition:...AssignmentExpression
  1. Let exprValue be the result of evaluatingAssignmentExpression.
  2. Let fromValue be ? GetValue(exprValue).
  3. Let excludedNames be a new emptyList.
  4. Return ? CopyDataProperties(object, fromValue, excludedNames).
PropertyDefinition:IdentifierReference
  1. Let propName beStringValueofIdentifierReference.
  2. Let exprValue be the result of evaluatingIdentifierReference.
  3. Let propValue be ? GetValue(exprValue).
  4. Assert: object is an ordinary, extensible object with no non-configurable properties.
  5. Return ! CreateDataPropertyOrThrow(object, propName, propValue).
PropertyDefinition:PropertyName:AssignmentExpression
  1. Let propKey be the result of evaluatingPropertyName.
  2. ReturnIfAbrupt(propKey).
  3. If thisPropertyDefinitionis contained within aScriptthat is being evaluated for JSON.parse (see step7ofJSON.parse), then
    1. Let isProtoSetter befalse.
  4. Else if propKey is the String value"__proto__"and ifIsComputedPropertyKeyofPropertyNameisfalse, then
    1. Let isProtoSetter betrue.
  5. Else,
    1. Let isProtoSetter befalse.
  6. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrueand isProtoSetter isfalse, then
    1. Let propValue be ?NamedEvaluationofAssignmentExpressionwith argument propKey.
  7. Else,
    1. Let exprValueRef be the result of evaluatingAssignmentExpression.
    2. Let propValue be ? GetValue(exprValueRef).
  8. If isProtoSetter istrue, then
    1. IfType(propValue) is either Object or Null, then
      1. Return ! object.[[SetPrototypeOf]](propValue).
    2. ReturnNormalCompletion(empty).
  9. Assert: object is an ordinary, extensible object with no non-configurable properties.
  10. Return ! CreateDataPropertyOrThrow(object, propKey, propValue).
PropertyDefinition:MethodDefinition
  1. Return ?MethodDefinitionEvaluationofMethodDefinitionwith arguments object andtrue.

13.2.6 Function Defining Expressions

See15.2forPrimaryExpression:FunctionExpression.

See15.5forPrimaryExpression:GeneratorExpression.

See15.7forPrimaryExpression:ClassExpression.

See15.8forPrimaryExpression:AsyncFunctionExpression.

See15.6forPrimaryExpression:AsyncGeneratorExpression.

13.2.7 Regular Expression Literals

Syntax

See12.8.5.

13.2.7.1 Static Semantics: Early Errors

PrimaryExpression:RegularExpressionLiteral

13.2.7.2 Static Semantics: IsValidRegularExpressionLiteral ( literal )

The abstract operation IsValidRegularExpressionLiteral takes argument literal. It determines if its argument is a valid regular expression literal. It performs the following steps when called:

  1. Assert: literal is aRegularExpressionLiteral.
  2. IfFlagTextof literal contains any code points other than g, i, m, s, u, or y, or if it contains the same code point more than once, returnfalse.
  3. Let patternText beBodyTextof literal.
  4. IfFlagTextof literal contains u, let u betrue; else let u befalse.
  5. If u isfalse, then
    1. Let stringValue beCodePointsToString(patternText).
    2. Set patternText to the sequence of code points resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
  6. Let parseResult beParsePattern(patternText, u).
  7. If parseResult is aParse Node, returntrue; else returnfalse.

13.2.7.3 Runtime Semantics: Evaluation

PrimaryExpression:RegularExpressionLiteral
  1. Let pattern be ! CodePointsToString(BodyTextofRegularExpressionLiteral).
  2. Let flags be ! CodePointsToString(FlagTextofRegularExpressionLiteral).
  3. ReturnRegExpCreate(pattern, flags).

13.2.8 Template Literals

Syntax

TemplateLiteral[Yield, Await, Tagged]:NoSubstitutionTemplateSubstitutionTemplate[?Yield, ?Await, ?Tagged]SubstitutionTemplate[Yield, Await, Tagged]:TemplateHeadExpression[+In, ?Yield, ?Await]TemplateSpans[?Yield, ?Await, ?Tagged]TemplateSpans[Yield, Await, Tagged]:TemplateTailTemplateMiddleList[?Yield, ?Await, ?Tagged]TemplateTailTemplateMiddleList[Yield, Await, Tagged]:TemplateMiddleExpression[+In, ?Yield, ?Await]TemplateMiddleList[?Yield, ?Await, ?Tagged]TemplateMiddleExpression[+In, ?Yield, ?Await]

13.2.8.1 Static Semantics: Early Errors

TemplateLiteral[Yield, Await, Tagged]:NoSubstitutionTemplateTemplateLiteral[Yield, Await, Tagged]:SubstitutionTemplate[?Yield, ?Await, ?Tagged]SubstitutionTemplate[Yield, Await, Tagged]:TemplateHeadExpression[+In, ?Yield, ?Await]TemplateSpans[?Yield, ?Await, ?Tagged]TemplateSpans[Yield, Await, Tagged]:TemplateTailTemplateMiddleList[Yield, Await, Tagged]:TemplateMiddleExpression[+In, ?Yield, ?Await]TemplateMiddleList[?Yield, ?Await, ?Tagged]TemplateMiddleExpression[+In, ?Yield, ?Await]

13.2.8.2 Static Semantics: TemplateStrings

With parameter raw.

TemplateLiteral:NoSubstitutionTemplate
  1. If raw isfalse, then
    1. Let string be theTVofNoSubstitutionTemplate.
  2. Else,
    1. Let string be theTRVofNoSubstitutionTemplate.
  3. Return aListwhose sole element is string.
SubstitutionTemplate:TemplateHeadExpressionTemplateSpans
  1. If raw isfalse, then
    1. Let head be theTVofTemplateHead.
  2. Else,
    1. Let head be theTRVofTemplateHead.
  3. Let tail beTemplateStringsofTemplateSpanswith argument raw.
  4. Return thelist-concatenationof « head » and tail.
TemplateSpans:TemplateTail
  1. If raw isfalse, then
    1. Let tail be theTVofTemplateTail.
  2. Else,
    1. Let tail be theTRVofTemplateTail.
  3. Return aListwhose sole element is tail.
TemplateSpans:TemplateMiddleListTemplateTail
  1. Let middle beTemplateStringsofTemplateMiddleListwith argument raw.
  2. If raw isfalse, then
    1. Let tail be theTVofTemplateTail.
  3. Else,
    1. Let tail be theTRVofTemplateTail.
  4. Return thelist-concatenationof middle and « tail ».
TemplateMiddleList:TemplateMiddleExpression
  1. If raw isfalse, then
    1. Let string be theTVofTemplateMiddle.
  2. Else,
    1. Let string be theTRVofTemplateMiddle.
  3. Return aListwhose sole element is string.
TemplateMiddleList:TemplateMiddleListTemplateMiddleExpression
  1. Let front beTemplateStringsofTemplateMiddleListwith argument raw.
  2. If raw isfalse, then
    1. Let last be theTVofTemplateMiddle.
  3. Else,
    1. Let last be theTRVofTemplateMiddle.
  4. Return thelist-concatenationof front and « last ».

13.2.8.3 GetTemplateObject ( templateLiteral )

The abstract operation GetTemplateObject takes argument templateLiteral (aParse Node). It performs the following steps when called:

  1. Let realm bethe current Realm Record.
  2. Let templateRegistry be realm.[[TemplateMap]].
  3. For each element e of templateRegistry, do
    1. If e.[[Site]] isthe same Parse Nodeas templateLiteral, then
      1. Return e.[[Array]].
  4. Let rawStrings beTemplateStringsof templateLiteral with argumenttrue.
  5. Let cookedStrings beTemplateStringsof templateLiteral with argumentfalse.
  6. Let count be the number of elements in theListcookedStrings.
  7. Assert: count ≤ 232 - 1.
  8. Let template be ! ArrayCreate(count).
  9. Let rawObj be ! ArrayCreate(count).
  10. Let index be 0.
  11. Repeat, while index < count,
    1. Let prop be ! ToString(𝔽(index)).
    2. Let cookedValue be cookedStrings[index].
    3. Perform ! DefinePropertyOrThrow(template, prop, PropertyDescriptor { [[Value]]: cookedValue, [[Writable]]:false, [[Enumerable]]:true, [[Configurable]]:false}).
    4. Let rawValue be the String value rawStrings[index].
    5. Perform ! DefinePropertyOrThrow(rawObj, prop, PropertyDescriptor { [[Value]]: rawValue, [[Writable]]:false, [[Enumerable]]:true, [[Configurable]]:false}).
    6. Set index to index + 1.
  12. Perform ! SetIntegrityLevel(rawObj,frozen).
  13. Perform ! DefinePropertyOrThrow(template,"raw", PropertyDescriptor { [[Value]]: rawObj, [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}).
  14. Perform ! SetIntegrityLevel(template,frozen).
  15. Append theRecord{ [[Site]]: templateLiteral, [[Array]]: template } to templateRegistry.
  16. Return template.
Note 1

The creation of a template object cannot result in anabrupt completion.

Note 2

EachTemplateLiteralin the program code of arealmis associated with a unique template object that is used in the evaluation of tagged Templates (13.2.8.5). The template objects are frozen and the same template object is used each time a specific tagged Template is evaluated. Whether template objects are created lazily upon first evaluation of theTemplateLiteralor eagerly prior to first evaluation is an implementation choice that is not observable to ECMAScript code.

Note 3

Future editions of this specification may define additional non-enumerable properties of template objects.

13.2.8.4 Runtime Semantics: SubstitutionEvaluation

TemplateSpans:TemplateTail
  1. Return a new emptyList.
TemplateSpans:TemplateMiddleListTemplateTail
  1. Return the result ofSubstitutionEvaluationofTemplateMiddleList.
TemplateMiddleList:TemplateMiddleExpression
  1. Let subRef be the result of evaluatingExpression.
  2. Let sub be ? GetValue(subRef).
  3. Return aListwhose sole element is sub.
TemplateMiddleList:TemplateMiddleListTemplateMiddleExpression
  1. Let preceding be ?SubstitutionEvaluationofTemplateMiddleList.
  2. Let nextRef be the result of evaluatingExpression.
  3. Let next be ? GetValue(nextRef).
  4. Return thelist-concatenationof preceding and « next ».

13.2.8.5 Runtime Semantics: Evaluation

TemplateLiteral:NoSubstitutionTemplate
  1. Return theTVofNoSubstitutionTemplateas defined in12.8.6.
SubstitutionTemplate:TemplateHeadExpressionTemplateSpans
  1. Let head be theTVofTemplateHeadas defined in12.8.6.
  2. Let subRef be the result of evaluatingExpression.
  3. Let sub be ? GetValue(subRef).
  4. Let middle be ? ToString(sub).
  5. Let tail be the result of evaluatingTemplateSpans.
  6. ReturnIfAbrupt(tail).
  7. Return thestring-concatenationof head, middle, and tail.
Note 1

The string conversion semantics applied to theExpressionvalue are like String.prototype.concat rather than the + operator.

TemplateSpans:TemplateTail
  1. Return theTVofTemplateTailas defined in12.8.6.
TemplateSpans:TemplateMiddleListTemplateTail
  1. Let head be the result of evaluatingTemplateMiddleList.
  2. ReturnIfAbrupt(head).
  3. Let tail be theTVofTemplateTailas defined in12.8.6.
  4. Return thestring-concatenationof head and tail.
TemplateMiddleList:TemplateMiddleExpression
  1. Let head be theTVofTemplateMiddleas defined in12.8.6.
  2. Let subRef be the result of evaluatingExpression.
  3. Let sub be ? GetValue(subRef).
  4. Let middle be ? ToString(sub).
  5. Return thestring-concatenationof head and middle.
Note 2

The string conversion semantics applied to theExpressionvalue are like String.prototype.concat rather than the + operator.

TemplateMiddleList:TemplateMiddleListTemplateMiddleExpression
  1. Let rest be the result of evaluatingTemplateMiddleList.
  2. ReturnIfAbrupt(rest).
  3. Let middle be theTVofTemplateMiddleas defined in12.8.6.
  4. Let subRef be the result of evaluatingExpression.
  5. Let sub be ? GetValue(subRef).
  6. Let last be ? ToString(sub).
  7. Return thestring-concatenationof rest, middle, and last.
Note 3

The string conversion semantics applied to theExpressionvalue are like String.prototype.concat rather than the + operator.

13.2.9 The Grouping Operator

13.2.9.1 Static Semantics: Early Errors

PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList

13.2.9.2 Runtime Semantics: Evaluation

PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. Return the result of evaluating expr.
ParenthesizedExpression:(Expression)
  1. Return the result of evaluatingExpression. This may be of type Reference.
Note

This algorithm does not applyGetValueto the result of evaluatingExpression. The principal motivation for this is so that operators such as delete and typeof may be applied to parenthesized expressions.

13.3 Left-Hand-Side Expressions

Syntax

MemberExpression[Yield, Await]:PrimaryExpression[?Yield, ?Await]MemberExpression[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]MemberExpression[?Yield, ?Await].IdentifierNameMemberExpression[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]SuperProperty[?Yield, ?Await]MetaPropertynewMemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]MemberExpression[?Yield, ?Await].PrivateIdentifierSuperProperty[Yield, Await]:super[Expression[+In, ?Yield, ?Await]]super.IdentifierNameMetaProperty:NewTargetImportMetaNewTarget:new.targetImportMeta:import.metaNewExpression[Yield, Await]:MemberExpression[?Yield, ?Await]newNewExpression[?Yield, ?Await]CallExpression[Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await]SuperCall[?Yield, ?Await]ImportCall[?Yield, ?Await]CallExpression[?Yield, ?Await]Arguments[?Yield, ?Await]CallExpression[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]CallExpression[?Yield, ?Await].IdentifierNameCallExpression[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]CallExpression[?Yield, ?Await].PrivateIdentifierSuperCall[Yield, Await]:superArguments[?Yield, ?Await]ImportCall[Yield, Await]:import(AssignmentExpression[+In, ?Yield, ?Await])Arguments[Yield, Await]:()(ArgumentList[?Yield, ?Await])(ArgumentList[?Yield, ?Await],)ArgumentList[Yield, Await]:AssignmentExpression[+In, ?Yield, ?Await]...AssignmentExpression[+In, ?Yield, ?Await]ArgumentList[?Yield, ?Await],AssignmentExpression[+In, ?Yield, ?Await]ArgumentList[?Yield, ?Await],...AssignmentExpression[+In, ?Yield, ?Await]OptionalExpression[Yield, Await]:MemberExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]CallExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]OptionalExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]OptionalChain[Yield, Await]:?.Arguments[?Yield, ?Await]?.[Expression[+In, ?Yield, ?Await]]?.IdentifierName?.TemplateLiteral[?Yield, ?Await, +Tagged]?.PrivateIdentifierOptionalChain[?Yield, ?Await]Arguments[?Yield, ?Await]OptionalChain[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]OptionalChain[?Yield, ?Await].IdentifierNameOptionalChain[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]OptionalChain[?Yield, ?Await].PrivateIdentifierLeftHandSideExpression[Yield, Await]:NewExpression[?Yield, ?Await]CallExpression[?Yield, ?Await]OptionalExpression[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
CallExpression:CoverCallExpressionAndAsyncArrowHead
the interpretation ofCoverCallExpressionAndAsyncArrowHeadis refined using the following grammar:

CallMemberExpression[Yield, Await]:MemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]

13.3.1 Static Semantics

13.3.1.1 Static Semantics: Early Errors

OptionalChain:?.TemplateLiteralOptionalChainTemplateLiteral
  • It is a Syntax Error if any code matches this production.
Note

This production exists in order to prevent automatic semicolon insertion rules (12.9) from being applied to the following code:

a?.b
`c`

so that it would be interpreted as two valid statements. The purpose is to maintain consistency with similar code without optional chaining:

a.b
`c`

which is a valid statement and where automatic semicolon insertion does not apply.

ImportMeta:import.meta

13.3.2 Property Accessors

Note

Properties are accessed by name, using either the dot notation:

or the bracket notation:

The dot notation is explained by the following syntactic conversion:

is identical in its behaviour to

MemberExpression[ <identifier-name-string> ]

and similarly

is identical in its behaviour to

CallExpression[ <identifier-name-string> ]

where <identifier-name-string> is the result of evaluatingStringValueofIdentifierName.

13.3.2.1 Runtime Semantics: Evaluation

MemberExpression:MemberExpression[Expression]
  1. Let baseReference be the result of evaluatingMemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the code matched by thisMemberExpressionisstrict mode code, let strict betrue; else let strict befalse.
  4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,Expression, strict).
MemberExpression:MemberExpression.IdentifierName
  1. Let baseReference be the result of evaluatingMemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the code matched by thisMemberExpressionisstrict mode code, let strict betrue; else let strict befalse.
  4. Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue,IdentifierName, strict).
MemberExpression:MemberExpression.PrivateIdentifier
  1. Let baseReference be the result of evaluatingMemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. Let bv be ? RequireObjectCoercible(baseValue).
  4. Let fieldNameString be theStringValueofPrivateIdentifier.
  5. Return ! MakePrivateReference(bv, fieldNameString).
CallExpression:CallExpression[Expression]
  1. Let baseReference be the result of evaluatingCallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the code matched by thisCallExpressionisstrict mode code, let strict betrue; else let strict befalse.
  4. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,Expression, strict).
CallExpression:CallExpression.IdentifierName
  1. Let baseReference be the result of evaluatingCallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If the code matched by thisCallExpressionisstrict mode code, let strict betrue; else let strict befalse.
  4. Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue,IdentifierName, strict).
CallExpression:CallExpression.PrivateIdentifier
  1. Let baseReference be the result of evaluatingCallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. Let bv be ? RequireObjectCoercible(baseValue).
  4. Let fieldNameString be theStringValueofPrivateIdentifier.
  5. Return ! MakePrivateReference(bv, fieldNameString).

13.3.3 EvaluatePropertyAccessWithExpressionKey ( baseValue, expression, strict )

The abstract operation EvaluatePropertyAccessWithExpressionKey takes arguments baseValue (anECMAScript language value), expression (aParse Node), and strict (a Boolean). It performs the following steps when called:

  1. Let propertyNameReference be the result of evaluating expression.
  2. Let propertyNameValue be ? GetValue(propertyNameReference).
  3. Let bv be ? RequireObjectCoercible(baseValue).
  4. Let propertyKey be ? ToPropertyKey(propertyNameValue).
  5. Return theReference Record{ [[Base]]: bv, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]:empty}.

13.3.4 EvaluatePropertyAccessWithIdentifierKey ( baseValue, identifierName, strict )

The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (anECMAScript language value), identifierName (aParse Node), and strict (a Boolean). It performs the following steps when called:

  1. Assert: identifierName is anIdentifierName.
  2. Let bv be ? RequireObjectCoercible(baseValue).
  3. Let propertyNameString beStringValueof identifierName.
  4. Return theReference Record{ [[Base]]: bv, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]:empty}.

13.3.5 The new Operator

13.3.5.1 Runtime Semantics: Evaluation

NewExpression:newNewExpression
  1. Return ? EvaluateNew(NewExpression,empty).
MemberExpression:newMemberExpressionArguments
  1. Return ? EvaluateNew(MemberExpression,Arguments).

13.3.5.1.1 EvaluateNew ( constructExpr, arguments )

The abstract operation EvaluateNew takes arguments constructExpr and arguments. It performs the following steps when called:

  1. Assert: constructExpr is either aNewExpressionor aMemberExpression.
  2. Assert: arguments is eitheremptyor anArguments.
  3. Let ref be the result of evaluating constructExpr.
  4. Let constructor be ? GetValue(ref).
  5. If arguments isempty, let argList be a new emptyList.
  6. Else,
    1. Let argList be ?ArgumentListEvaluationof arguments.
  7. IfIsConstructor(constructor) isfalse, throw aTypeErrorexception.
  8. Return ? Construct(constructor, argList).

13.3.6 Function Calls

13.3.6.1 Runtime Semantics: Evaluation

CallExpression:CoverCallExpressionAndAsyncArrowHead
  1. Let expr be theCallMemberExpressionthat iscoveredbyCoverCallExpressionAndAsyncArrowHead.
  2. Let memberExpr be theMemberExpressionof expr.
  3. Let arguments be theArgumentsof expr.
  4. Let ref be the result of evaluating memberExpr.
  5. Let func be ? GetValue(ref).
  6. If ref is aReference Record,IsPropertyReference(ref) isfalse, and ref.[[ReferencedName]] is"eval", then
    1. IfSameValue(func,%eval%) istrue, then
      1. Let argList be ?ArgumentListEvaluationof arguments.
      2. If argList has no elements, returnundefined.
      3. Let evalArg be the first element of argList.
      4. If the source code matching thisCallExpressionisstrict mode code, let strictCaller betrue. Otherwise let strictCaller befalse.
      5. Let evalRealm bethe current Realm Record.
      6. Return ? PerformEval(evalArg, evalRealm, strictCaller,true).
  7. Let thisCall be thisCallExpression.
  8. Let tailCall beIsInTailPosition(thisCall).
  9. Return ? EvaluateCall(func, ref, arguments, tailCall).

ACallExpressionevaluation that executes step6.a.viis a direct eval.

CallExpression:CallExpressionArguments
  1. Let ref be the result of evaluatingCallExpression.
  2. Let func be ? GetValue(ref).
  3. Let thisCall be thisCallExpression.
  4. Let tailCall beIsInTailPosition(thisCall).
  5. Return ? EvaluateCall(func, ref,Arguments, tailCall).

13.3.6.2 EvaluateCall ( func, ref, arguments, tailPosition )

The abstract operation EvaluateCall takes arguments func (anECMAScript language value), ref (anECMAScript language valueor aReference Record), arguments (aParse Node), and tailPosition (a Boolean). It performs the following steps when called:

  1. If ref is aReference Record, then
    1. IfIsPropertyReference(ref) istrue, then
      1. Let thisValue beGetThisValue(ref).
    2. Else,
      1. Let refEnv be ref.[[Base]].
      2. Assert: refEnv is anEnvironment Record.
      3. Let thisValue be refEnv.WithBaseObject().
  2. Else,
    1. Let thisValue beundefined.
  3. Let argList be ?ArgumentListEvaluationof arguments.
  4. IfType(func) is not Object, throw aTypeErrorexception.
  5. IfIsCallable(func) isfalse, throw aTypeErrorexception.
  6. If tailPosition istrue, performPrepareForTailCall().
  7. Let result beCall(func, thisValue, argList).
  8. Assert: If tailPosition istrue, the above call will not return here, but instead evaluation will continue as if the following return has already occurred.
  9. Assert: If result is not anabrupt completion, thenType(result) is anECMAScript language type.
  10. Return result.

13.3.7 The super Keyword

13.3.7.1 Runtime Semantics: Evaluation

SuperProperty:super[Expression]
  1. Let env beGetThisEnvironment().
  2. Let actualThis be ? env.GetThisBinding().
  3. Let propertyNameReference be the result of evaluatingExpression.
  4. Let propertyNameValue be ? GetValue(propertyNameReference).
  5. Let propertyKey be ? ToPropertyKey(propertyNameValue).
  6. If the code matched by thisSuperPropertyisstrict mode code, let strict betrue; else let strict befalse.
  7. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
SuperProperty:super.IdentifierName
  1. Let env beGetThisEnvironment().
  2. Let actualThis be ? env.GetThisBinding().
  3. Let propertyKey beStringValueofIdentifierName.
  4. If the code matched by thisSuperPropertyisstrict mode code, let strict betrue; else let strict befalse.
  5. Return ? MakeSuperPropertyReference(actualThis, propertyKey, strict).
SuperCall:superArguments
  1. Let newTarget beGetNewTarget().
  2. Assert:Type(newTarget) is Object.
  3. Let func be ! GetSuperConstructor().
  4. Let argList be ?ArgumentListEvaluationofArguments.
  5. IfIsConstructor(func) isfalse, throw aTypeErrorexception.
  6. Let result be ? Construct(func, argList, newTarget).
  7. Let thisER beGetThisEnvironment().
  8. Perform ? thisER.BindThisValue(result).
  9. Let F be thisER.[[FunctionObject]].
  10. Assert: F is an ECMAScriptfunction object.
  11. Perform ? InitializeInstanceElements(result, F).
  12. Return result.

13.3.7.2 GetSuperConstructor ( )

The abstract operation GetSuperConstructor takes no arguments. It performs the following steps when called:

  1. Let envRec beGetThisEnvironment().
  2. Assert: envRec is afunction Environment Record.
  3. Let activeFunction be envRec.[[FunctionObject]].
  4. Assert: activeFunction is an ECMAScriptfunction object.
  5. Let superConstructor be ! activeFunction.[[GetPrototypeOf]]().
  6. Return superConstructor.

13.3.7.3 MakeSuperPropertyReference ( actualThis, propertyKey, strict )

The abstract operation MakeSuperPropertyReference takes arguments actualThis, propertyKey, and strict. It performs the following steps when called:

  1. Let env beGetThisEnvironment().
  2. Assert: env.HasSuperBinding() istrue.
  3. Let baseValue be ? env.GetSuperBase().
  4. Let bv be ? RequireObjectCoercible(baseValue).
  5. Return theReference Record{ [[Base]]: bv, [[ReferencedName]]: propertyKey, [[Strict]]: strict, [[ThisValue]]: actualThis }.
  6. NOTE: This returns aSuper Reference Record.

13.3.8 Argument Lists

Note

The evaluation of an argument list produces aListof values.

13.3.8.1 Runtime Semantics: ArgumentListEvaluation

Arguments:()
  1. Return a new emptyList.
ArgumentList:AssignmentExpression
  1. Let ref be the result of evaluatingAssignmentExpression.
  2. Let arg be ? GetValue(ref).
  3. Return aListwhose sole element is arg.
ArgumentList:...AssignmentExpression
  1. Let list be a new emptyList.
  2. Let spreadRef be the result of evaluatingAssignmentExpression.
  3. Let spreadObj be ? GetValue(spreadRef).
  4. Let iteratorRecord be ? GetIterator(spreadObj).
  5. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return list.
    3. Let nextArg be ? IteratorValue(next).
    4. Append nextArg as the last element of list.
ArgumentList:ArgumentList,AssignmentExpression
  1. Let precedingArgs be ?ArgumentListEvaluationofArgumentList.
  2. Let ref be the result of evaluatingAssignmentExpression.
  3. Let arg be ? GetValue(ref).
  4. Return thelist-concatenationof precedingArgs and « arg ».
ArgumentList:ArgumentList,...AssignmentExpression
  1. Let precedingArgs be ?ArgumentListEvaluationofArgumentList.
  2. Let spreadRef be the result of evaluatingAssignmentExpression.
  3. Let iteratorRecord be ? GetIterator(?GetValue(spreadRef)).
  4. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return precedingArgs.
    3. Let nextArg be ? IteratorValue(next).
    4. Append nextArg as the last element of precedingArgs.
TemplateLiteral:NoSubstitutionTemplate
  1. Let templateLiteral be thisTemplateLiteral.
  2. Let siteObj beGetTemplateObject(templateLiteral).
  3. Return aListwhose sole element is siteObj.
TemplateLiteral:SubstitutionTemplate
  1. Let templateLiteral be thisTemplateLiteral.
  2. Let siteObj beGetTemplateObject(templateLiteral).
  3. Let remaining be ?ArgumentListEvaluationofSubstitutionTemplate.
  4. Return thelist-concatenationof « siteObj » and remaining.
SubstitutionTemplate:TemplateHeadExpressionTemplateSpans
  1. Let firstSubRef be the result of evaluatingExpression.
  2. Let firstSub be ? GetValue(firstSubRef).
  3. Let restSub be ?SubstitutionEvaluationofTemplateSpans.
  4. Assert: restSub is a possibly emptyList.
  5. Return thelist-concatenationof « firstSub » and restSub.

13.3.9 Optional Chains

Note
An optional chain is a chain of one or more property accesses and function calls, the first of which begins with the token ?..

13.3.9.1 Runtime Semantics: Evaluation

OptionalExpression:MemberExpressionOptionalChain
  1. Let baseReference be the result of evaluatingMemberExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue isundefinedornull, then
    1. Returnundefined.
  4. Return the result of performingChainEvaluationofOptionalChainwith arguments baseValue and baseReference.
OptionalExpression:CallExpressionOptionalChain
  1. Let baseReference be the result of evaluatingCallExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue isundefinedornull, then
    1. Returnundefined.
  4. Return the result of performingChainEvaluationofOptionalChainwith arguments baseValue and baseReference.
OptionalExpression:OptionalExpressionOptionalChain
  1. Let baseReference be the result of evaluatingOptionalExpression.
  2. Let baseValue be ? GetValue(baseReference).
  3. If baseValue isundefinedornull, then
    1. Returnundefined.
  4. Return the result of performingChainEvaluationofOptionalChainwith arguments baseValue and baseReference.

13.3.9.2 Runtime Semantics: ChainEvaluation

With parameters baseValue and baseReference.

OptionalChain:?.Arguments
  1. Let thisChain be thisOptionalChain.
  2. Let tailCall beIsInTailPosition(thisChain).
  3. Return ? EvaluateCall(baseValue, baseReference,Arguments, tailCall).
OptionalChain:?.[Expression]
  1. If the code matched by thisOptionalChainisstrict mode code, let strict betrue; else let strict befalse.
  2. Return ? EvaluatePropertyAccessWithExpressionKey(baseValue,Expression, strict).
OptionalChain:?.IdentifierName
  1. If the code matched by thisOptionalChainisstrict mode code, let strict betrue; else let strict befalse.
  2. Return ? EvaluatePropertyAccessWithIdentifierKey(baseValue,IdentifierName, strict).
OptionalChain:?.PrivateIdentifier
  1. Let bv be ? RequireObjectCoercible(baseValue).
  2. Let fieldNameString be theStringValueofPrivateIdentifier.
  3. Return ! MakePrivateReference(bv, fieldNameString).
OptionalChain:OptionalChainArguments
  1. Let optionalChain beOptionalChain.
  2. Let newReference be ?ChainEvaluationof optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. Let thisChain be thisOptionalChain.
  5. Let tailCall beIsInTailPosition(thisChain).
  6. Return ? EvaluateCall(newValue, newReference,Arguments, tailCall).
OptionalChain:OptionalChain[Expression]
  1. Let optionalChain beOptionalChain.
  2. Let newReference be ?ChainEvaluationof optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. If the code matched by thisOptionalChainisstrict mode code, let strict betrue; else let strict befalse.
  5. Return ? EvaluatePropertyAccessWithExpressionKey(newValue,Expression, strict).
OptionalChain:OptionalChain.IdentifierName
  1. Let optionalChain beOptionalChain.
  2. Let newReference be ?ChainEvaluationof optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. If the code matched by thisOptionalChainisstrict mode code, let strict betrue; else let strict befalse.
  5. Return ? EvaluatePropertyAccessWithIdentifierKey(newValue,IdentifierName, strict).
OptionalChain:OptionalChain.PrivateIdentifier
  1. Let optionalChain beOptionalChain.
  2. Let newReference be ?ChainEvaluationof optionalChain with arguments baseValue and baseReference.
  3. Let newValue be ? GetValue(newReference).
  4. Let nv be ? RequireObjectCoercible(newValue).
  5. Let fieldNameString be theStringValueofPrivateIdentifier.
  6. Return ! MakePrivateReference(nv, fieldNameString).

13.3.10 Import Calls

13.3.10.1 Runtime Semantics: Evaluation

ImportCall:import(AssignmentExpression)
  1. Let referencingScriptOrModule be ! GetActiveScriptOrModule().
  2. Let argRef be the result of evaluatingAssignmentExpression.
  3. Let specifier be ? GetValue(argRef).
  4. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  5. Let specifierString beToString(specifier).
  6. IfAbruptRejectPromise(specifierString, promiseCapability).
  7. Perform ! HostImportModuleDynamically(referencingScriptOrModule, specifierString, promiseCapability).
  8. Return promiseCapability.[[Promise]].

13.3.11 Tagged Templates

Note

A tagged template is a function call where the arguments of the call are derived from aTemplateLiteral(13.2.8). The actual arguments include a template object (13.2.8.3) and the values produced by evaluating the expressions embedded within theTemplateLiteral.

13.3.11.1 Runtime Semantics: Evaluation

MemberExpression:MemberExpressionTemplateLiteral
  1. Let tagRef be the result of evaluatingMemberExpression.
  2. Let tagFunc be ? GetValue(tagRef).
  3. Let thisCall be thisMemberExpression.
  4. Let tailCall beIsInTailPosition(thisCall).
  5. Return ? EvaluateCall(tagFunc, tagRef,TemplateLiteral, tailCall).
CallExpression:CallExpressionTemplateLiteral
  1. Let tagRef be the result of evaluatingCallExpression.
  2. Let tagFunc be ? GetValue(tagRef).
  3. Let thisCall be thisCallExpression.
  4. Let tailCall beIsInTailPosition(thisCall).
  5. Return ? EvaluateCall(tagFunc, tagRef,TemplateLiteral, tailCall).

13.3.12 Meta Properties

13.3.12.1 Runtime Semantics: Evaluation

NewTarget:new.target
  1. ReturnGetNewTarget().
ImportMeta:import.meta
  1. Let module be ! GetActiveScriptOrModule().
  2. Assert: module is aSource Text Module Record.
  3. Let importMeta be module.[[ImportMeta]].
  4. If importMeta isempty, then
    1. Set importMeta to ! OrdinaryObjectCreate(null).
    2. Let importMetaValues be ! HostGetImportMetaProperties(module).
    3. For eachRecord{ [[Key]], [[Value]] } p of importMetaValues, do
      1. Perform ! CreateDataPropertyOrThrow(importMeta, p.[[Key]], p.[[Value]]).
    4. Perform ! HostFinalizeImportMeta(importMeta, module).
    5. Set module.[[ImportMeta]] to importMeta.
    6. Return importMeta.
  5. Else,
    1. Assert:Type(importMeta) is Object.
    2. Return importMeta.

13.3.12.1.1 HostGetImportMetaProperties ( moduleRecord )

Thehost-definedabstract operation HostGetImportMetaProperties takes argument moduleRecord (aModule Record). It allows hosts to provide property keys and values for the object returned from import.meta.

An implementation of HostGetImportMetaProperties must conform to the following requirements:

  • It must return anormal completionwith a value of aListwhose values are all Records with two fields, [[Key]] and [[Value]].
  • Each suchRecord's [[Key]] field must be a property key, i.e.,IsPropertyKeymust returntruewhen applied to it.
  • Each suchRecord's [[Value]] field must be an ECMAScript value.

The default implementation of HostGetImportMetaProperties is to returnNormalCompletion(« »).

13.3.12.1.2 HostFinalizeImportMeta ( importMeta, moduleRecord )

Thehost-definedabstract operation HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (aModule Record). It allows hosts to perform any extraordinary operations to prepare the object returned from import.meta.

Most hosts will be able to simply defineHostGetImportMetaProperties, and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly manipulate the object before it is exposed to ECMAScript code.

An implementation of HostFinalizeImportMeta must conform to the following requirements:

The default implementation of HostFinalizeImportMeta is to returnNormalCompletion(empty).

13.4 Update Expressions

Syntax

UpdateExpression[Yield, Await]:LeftHandSideExpression[?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]++LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]--++UnaryExpression[?Yield, ?Await]--UnaryExpression[?Yield, ?Await]

13.4.1 Static Semantics: Early Errors

UpdateExpression:LeftHandSideExpression++LeftHandSideExpression--UpdateExpression:++UnaryExpression--UnaryExpression

13.4.2 Postfix Increment Operator

13.4.2.1 Runtime Semantics: Evaluation

UpdateExpression:LeftHandSideExpression++
  1. Let lhs be the result of evaluatingLeftHandSideExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(lhs)).
  3. Let newValue be ! Type(oldValue)::add(oldValue,Type(oldValue)::unit).
  4. Perform ? PutValue(lhs, newValue).
  5. Return oldValue.

13.4.3 Postfix Decrement Operator

13.4.3.1 Runtime Semantics: Evaluation

UpdateExpression:LeftHandSideExpression--
  1. Let lhs be the result of evaluatingLeftHandSideExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(lhs)).
  3. Let newValue be ! Type(oldValue)::subtract(oldValue,Type(oldValue)::unit).
  4. Perform ? PutValue(lhs, newValue).
  5. Return oldValue.

13.4.4 Prefix Increment Operator

13.4.4.1 Runtime Semantics: Evaluation

UpdateExpression:++UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(expr)).
  3. Let newValue be ! Type(oldValue)::add(oldValue,Type(oldValue)::unit).
  4. Perform ? PutValue(expr, newValue).
  5. Return newValue.

13.4.5 Prefix Decrement Operator

13.4.5.1 Runtime Semantics: Evaluation

UpdateExpression:--UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(expr)).
  3. Let newValue be ! Type(oldValue)::subtract(oldValue,Type(oldValue)::unit).
  4. Perform ? PutValue(expr, newValue).
  5. Return newValue.

13.5 Unary Operators

Syntax

UnaryExpression[Yield, Await]:UpdateExpression[?Yield, ?Await]deleteUnaryExpression[?Yield, ?Await]voidUnaryExpression[?Yield, ?Await]typeofUnaryExpression[?Yield, ?Await]+UnaryExpression[?Yield, ?Await]-UnaryExpression[?Yield, ?Await]~UnaryExpression[?Yield, ?Await]!UnaryExpression[?Yield, ?Await][+Await]AwaitExpression[?Yield]

13.5.1 The delete Operator

13.5.1.1 Static Semantics: Early Errors

UnaryExpression:deleteUnaryExpressionNote

The last rule means that expressions such as delete (((foo))) produce early errors because of recursive application of the first rule.

13.5.1.2 Runtime Semantics: Evaluation

UnaryExpression:deleteUnaryExpression
  1. Let ref be the result of evaluatingUnaryExpression.
  2. ReturnIfAbrupt(ref).
  3. If ref is not aReference Record, returntrue.
  4. IfIsUnresolvableReference(ref) istrue, then
    1. Assert: ref.[[Strict]] isfalse.
    2. Returntrue.
  5. IfIsPropertyReference(ref) istrue, then
    1. Assert: ! IsPrivateReference(ref) isfalse.
    2. IfIsSuperReference(ref) istrue, throw aReferenceErrorexception.
    3. Let baseObj be ! ToObject(ref.[[Base]]).
    4. Let deleteStatus be ? baseObj.[[Delete]](ref.[[ReferencedName]]).
    5. If deleteStatus isfalseand ref.[[Strict]] istrue, throw aTypeErrorexception.
    6. Return deleteStatus.
  6. Else,
    1. Let base be ref.[[Base]].
    2. Assert: base is anEnvironment Record.
    3. Return ? base.DeleteBinding(ref.[[ReferencedName]]).
Note 1

When a delete operator occurs withinstrict mode code, aSyntaxErrorexception is thrown if itsUnaryExpressionis a direct reference to a variable, function argument, or function name. In addition, if a delete operator occurs withinstrict mode codeand the property to be deleted has the attribute { [[Configurable]]:false} (or otherwise cannot be deleted), aTypeErrorexception is thrown.

Note 2

The object that may be created in step5.cis not accessible outside of the above abstract operation and theordinary object[[Delete]] internal method. An implementation might choose to avoid the actual creation of that object.

13.5.2 The void Operator

13.5.2.1 Runtime Semantics: Evaluation

UnaryExpression:voidUnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Perform ? GetValue(expr).
  3. Returnundefined.
Note

GetValuemust be called even though its value is not used because it may have observable side-effects.

13.5.3 The typeof Operator

13.5.3.1 Runtime Semantics: Evaluation

UnaryExpression:typeofUnaryExpression
  1. Let val be the result of evaluatingUnaryExpression.
  2. If val is aReference Record, then
    1. IfIsUnresolvableReference(val) istrue, return"undefined".
  3. Set val to ? GetValue(val).
  4. Return a String according toTable 41.
Table 41: typeof Operator Results
Type of valResult
Undefined"undefined"
Null"object"
Boolean"boolean"
Number"number"
String"string"
Symbol"symbol"
BigInt"bigint"
Object (does not implement [[Call]])"object"
Object (implements [[Call]])"function"
Note

An additional entry related to [[IsHTMLDDA]] Internal Slot can be found inB.3.6.3.

13.5.4 Unary + Operator

Note

The unary + operator converts its operand to Number type.

13.5.4.1 Runtime Semantics: Evaluation

UnaryExpression:+UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Return ? ToNumber(?GetValue(expr)).

13.5.5 Unary - Operator

Note

The unary - operator converts its operand to Number type and then negates it. Negating+0𝔽 produces-0𝔽, and negating-0𝔽 produces+0𝔽.

13.5.5.1 Runtime Semantics: Evaluation

UnaryExpression:-UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(expr)).
  3. Let T beType(oldValue).
  4. Return ! T::unaryMinus(oldValue).

13.5.6 Bitwise NOT Operator ( ~ )

13.5.6.1 Runtime Semantics: Evaluation

UnaryExpression:~UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Let oldValue be ? ToNumeric(?GetValue(expr)).
  3. Let T beType(oldValue).
  4. Return ! T::bitwiseNOT(oldValue).

13.5.7 Logical NOT Operator ( ! )

13.5.7.1 Runtime Semantics: Evaluation

UnaryExpression:!UnaryExpression
  1. Let expr be the result of evaluatingUnaryExpression.
  2. Let oldValue be ! ToBoolean(?GetValue(expr)).
  3. If oldValue istrue, returnfalse.
  4. Returntrue.

13.6 Exponentiation Operator

Syntax

ExponentiationExpression[Yield, Await]:UnaryExpression[?Yield, ?Await]UpdateExpression[?Yield, ?Await]**ExponentiationExpression[?Yield, ?Await]

13.6.1 Runtime Semantics: Evaluation

ExponentiationExpression:UpdateExpression**ExponentiationExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(UpdateExpression, **,ExponentiationExpression).

13.7 Multiplicative Operators

Syntax

MultiplicativeExpression[Yield, Await]:ExponentiationExpression[?Yield, ?Await]MultiplicativeExpression[?Yield, ?Await]MultiplicativeOperatorExponentiationExpression[?Yield, ?Await]MultiplicativeOperator:one of*/%Note
  • The * operator performs multiplication, producing the product of its operands.
  • The / operator performs division, producing the quotient of its operands.
  • The % operator yields the remainder of its operands from an implied division.

13.7.1 Runtime Semantics: Evaluation

MultiplicativeExpression:MultiplicativeExpressionMultiplicativeOperatorExponentiationExpression
  1. Let opText be the source text matched byMultiplicativeOperator.
  2. Return ? EvaluateStringOrNumericBinaryExpression(MultiplicativeExpression, opText,ExponentiationExpression).

13.8 Additive Operators

Syntax

AdditiveExpression[Yield, Await]:MultiplicativeExpression[?Yield, ?Await]AdditiveExpression[?Yield, ?Await]+MultiplicativeExpression[?Yield, ?Await]AdditiveExpression[?Yield, ?Await]-MultiplicativeExpression[?Yield, ?Await]

13.8.1 The Addition Operator ( + )

Note

The addition operator either performs string concatenation or numeric addition.

13.8.1.1 Runtime Semantics: Evaluation

AdditiveExpression:AdditiveExpression+MultiplicativeExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, +,MultiplicativeExpression).

13.8.2 The Subtraction Operator ( - )

Note

The - operator performs subtraction, producing the difference of its operands.

13.8.2.1 Runtime Semantics: Evaluation

AdditiveExpression:AdditiveExpression-MultiplicativeExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(AdditiveExpression, -,MultiplicativeExpression).

13.9 Bitwise Shift Operators

Syntax

ShiftExpression[Yield, Await]:AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]<<AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]>>AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]>>>AdditiveExpression[?Yield, ?Await]

13.9.1 The Left Shift Operator ( << )

Note

Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

13.9.1.1 Runtime Semantics: Evaluation

ShiftExpression:ShiftExpression<<AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, <<,AdditiveExpression).

13.9.2 The Signed Right Shift Operator ( >> )

Note

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.2.1 Runtime Semantics: Evaluation

ShiftExpression:ShiftExpression>>AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>,AdditiveExpression).

13.9.3 The Unsigned Right Shift Operator ( >>> )

Note

Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

13.9.3.1 Runtime Semantics: Evaluation

ShiftExpression:ShiftExpression>>>AdditiveExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(ShiftExpression, >>>,AdditiveExpression).

13.10 Relational Operators

Note 1

The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

RelationalExpression[In, Yield, Await]:ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]<ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]>ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]<=ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]>=ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]instanceofShiftExpression[?Yield, ?Await][+In]RelationalExpression[+In, ?Yield, ?Await]inShiftExpression[?Yield, ?Await][+In]PrivateIdentifierinShiftExpression[?Yield, ?Await]Note 2

The [In] grammar parameter is needed to avoid confusing the in operator in a relational expression with the in operator in a for statement.

13.10.1 Runtime Semantics: Evaluation

RelationalExpression:RelationalExpression<ShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(lval, rval,true).
  6. If r isundefined, returnfalse. Otherwise, return r.
RelationalExpression:RelationalExpression>ShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(rval, lval,false).
  6. If r isundefined, returnfalse. Otherwise, return r.
RelationalExpression:RelationalExpression<=ShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(rval, lval,false).
  6. If r istrueorundefined, returnfalse. Otherwise, returntrue.
RelationalExpression:RelationalExpression>=ShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLessThan(lval, rval,true).
  6. If r istrueorundefined, returnfalse. Otherwise, returntrue.
RelationalExpression:RelationalExpressioninstanceofShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. Return ? InstanceofOperator(lval, rval).
RelationalExpression:RelationalExpressioninShiftExpression
  1. Let lref be the result of evaluatingRelationalExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingShiftExpression.
  4. Let rval be ? GetValue(rref).
  5. IfType(rval) is not Object, throw aTypeErrorexception.
  6. Return ? HasProperty(rval, ? ToPropertyKey(lval)).
RelationalExpression:PrivateIdentifierinShiftExpression
  1. Let privateIdentifier be theStringValueofPrivateIdentifier.
  2. Let rref be the result of evaluatingShiftExpression.
  3. Let rval be ? GetValue(rref).
  4. IfType(rval) is not Object, throw aTypeErrorexception.
  5. Let privateEnv be therunning execution context's PrivateEnvironment.
  6. Let privateName be ! ResolvePrivateIdentifier(privateEnv, privateIdentifier).
  7. If ! PrivateElementFind(privateName, rval) is notempty, returntrue.
  8. Returnfalse.

13.10.2 InstanceofOperator ( V, target )

The abstract operation InstanceofOperator takes arguments V (anECMAScript language value) and target (anECMAScript language value). It implements the generic algorithm for determining if V is an instance of target either by consulting target's@@hasInstancemethod or, if absent, determining whether the value of target's"prototype"property is present in V's prototype chain. It performs the following steps when called:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let instOfHandler be ? GetMethod(target,@@hasInstance).
  3. If instOfHandler is notundefined, then
    1. Return ! ToBoolean(?Call(instOfHandler, target, « V »)).
  4. IfIsCallable(target) isfalse, throw aTypeErrorexception.
  5. Return ? OrdinaryHasInstance(target, V).
Note

Steps4and5provide compatibility with previous editions of ECMAScript that did not use a@@hasInstancemethod to define the instanceof operator semantics. If an object does not define or inherit@@hasInstanceit uses the default instanceof semantics.

13.11 Equality Operators

Note

The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationship named by the operator holds between its two operands.

Syntax

EqualityExpression[In, Yield, Await]:RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]==RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]!=RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]===RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]!==RelationalExpression[?In, ?Yield, ?Await]

13.11.1 Runtime Semantics: Evaluation

EqualityExpression:EqualityExpression==RelationalExpression
  1. Let lref be the result of evaluatingEqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingRelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. ReturnIsLooselyEqual(rval, lval).
EqualityExpression:EqualityExpression!=RelationalExpression
  1. Let lref be the result of evaluatingEqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingRelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ? IsLooselyEqual(rval, lval).
  6. If r istrue, returnfalse. Otherwise, returntrue.
EqualityExpression:EqualityExpression===RelationalExpression
  1. Let lref be the result of evaluatingEqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingRelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. ReturnIsStrictlyEqual(rval, lval).
EqualityExpression:EqualityExpression!==RelationalExpression
  1. Let lref be the result of evaluatingEqualityExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingRelationalExpression.
  4. Let rval be ? GetValue(rref).
  5. Let r be ! IsStrictlyEqual(rval, lval).
  6. If r istrue, returnfalse. Otherwise, returntrue.
Note 1

Given the above definition of equality:

  • String comparison can be forced by: `${a}` == `${b}`.
  • Numeric comparison can be forced by: +a == +b.
  • Boolean comparison can be forced by: !a == !b.
Note 2

The equality operators maintain the following invariants:

  • A != B is equivalent to !(A == B).
  • A == B is equivalent to B == A, except in the order of evaluation of A and B.
Note 3

The equality operator is not always transitive. For example, there might be two distinct String objects, each representing the same String value; each String object would be considered equal to the String value by the == operator, but the two String objects would not be equal to each other. For example:

  • new String("a") == "a" and "a" == new String("a") are bothtrue.
  • new String("a") == new String("a") isfalse.
Note 4

Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt to use the more complex, semantically oriented definitions of character or string equality and collating order defined in the Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as unequal. In effect this algorithm assumes that both Strings are already in normalized form.

13.12 Binary Bitwise Operators

Syntax

BitwiseANDExpression[In, Yield, Await]:EqualityExpression[?In, ?Yield, ?Await]BitwiseANDExpression[?In, ?Yield, ?Await]&EqualityExpression[?In, ?Yield, ?Await]BitwiseXORExpression[In, Yield, Await]:BitwiseANDExpression[?In, ?Yield, ?Await]BitwiseXORExpression[?In, ?Yield, ?Await]^BitwiseANDExpression[?In, ?Yield, ?Await]BitwiseORExpression[In, Yield, Await]:BitwiseXORExpression[?In, ?Yield, ?Await]BitwiseORExpression[?In, ?Yield, ?Await]|BitwiseXORExpression[?In, ?Yield, ?Await]

13.12.1 Runtime Semantics: Evaluation

BitwiseANDExpression:BitwiseANDExpression&EqualityExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseANDExpression, &,EqualityExpression).
BitwiseXORExpression:BitwiseXORExpression^BitwiseANDExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseXORExpression, ^,BitwiseANDExpression).
BitwiseORExpression:BitwiseORExpression|BitwiseXORExpression
  1. Return ? EvaluateStringOrNumericBinaryExpression(BitwiseORExpression, |,BitwiseXORExpression).

13.13 Binary Logical Operators

Syntax

LogicalANDExpression[In, Yield, Await]:BitwiseORExpression[?In, ?Yield, ?Await]LogicalANDExpression[?In, ?Yield, ?Await]&&BitwiseORExpression[?In, ?Yield, ?Await]LogicalORExpression[In, Yield, Await]:LogicalANDExpression[?In, ?Yield, ?Await]LogicalORExpression[?In, ?Yield, ?Await]||LogicalANDExpression[?In, ?Yield, ?Await]CoalesceExpression[In, Yield, Await]:CoalesceExpressionHead[?In, ?Yield, ?Await]??BitwiseORExpression[?In, ?Yield, ?Await]CoalesceExpressionHead[In, Yield, Await]:CoalesceExpression[?In, ?Yield, ?Await]BitwiseORExpression[?In, ?Yield, ?Await]ShortCircuitExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await]CoalesceExpression[?In, ?Yield, ?Await]Note

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.

13.13.1 Runtime Semantics: Evaluation

LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression
  1. Let lref be the result of evaluatingLogicalANDExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool isfalse, return lval.
  5. Let rref be the result of evaluatingBitwiseORExpression.
  6. Return ? GetValue(rref).
LogicalORExpression:LogicalORExpression||LogicalANDExpression
  1. Let lref be the result of evaluatingLogicalORExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool istrue, return lval.
  5. Let rref be the result of evaluatingLogicalANDExpression.
  6. Return ? GetValue(rref).
CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. Let lref be the result of evaluatingCoalesceExpressionHead.
  2. Let lval be ? GetValue(lref).
  3. If lval isundefinedornull, then
    1. Let rref be the result of evaluatingBitwiseORExpression.
    2. Return ? GetValue(rref).
  4. Otherwise, return lval.

13.14 Conditional Operator ( ? : )

Syntax

ConditionalExpression[In, Yield, Await]:ShortCircuitExpression[?In, ?Yield, ?Await]ShortCircuitExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[?In, ?Yield, ?Await]Note

The grammar for aConditionalExpressionin ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be anExpressionbut restrict the third expression to be aConditionalExpression. The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

13.14.1 Runtime Semantics: Evaluation

ConditionalExpression:ShortCircuitExpression?AssignmentExpression:AssignmentExpression
  1. Let lref be the result of evaluatingShortCircuitExpression.
  2. Let lval be ! ToBoolean(?GetValue(lref)).
  3. If lval istrue, then
    1. Let trueRef be the result of evaluating the firstAssignmentExpression.
    2. Return ? GetValue(trueRef).
  4. Else,
    1. Let falseRef be the result of evaluating the secondAssignmentExpression.
    2. Return ? GetValue(falseRef).

13.15 Assignment Operators

Syntax

AssignmentExpression[In, Yield, Await]:ConditionalExpression[?In, ?Yield, ?Await][+Yield]YieldExpression[?In, ?Await]ArrowFunction[?In, ?Yield, ?Await]AsyncArrowFunction[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]AssignmentOperatorAssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]&&=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]||=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]??=AssignmentExpression[?In, ?Yield, ?Await]AssignmentOperator:one of*=/=%=+=-=<<=>>=>>>=&=^=|=**=

13.15.1 Static Semantics: Early Errors

AssignmentExpression:LeftHandSideExpression=AssignmentExpression

IfLeftHandSideExpressionis anObjectLiteralor anArrayLiteral, the following Early Error rules are applied:

IfLeftHandSideExpressionis neither anObjectLiteralnor anArrayLiteral, the following Early Error rule is applied:

AssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpressionLeftHandSideExpression&&=AssignmentExpressionLeftHandSideExpression||=AssignmentExpressionLeftHandSideExpression??=AssignmentExpression

13.15.2 Runtime Semantics: Evaluation

AssignmentExpression:LeftHandSideExpression=AssignmentExpression
  1. IfLeftHandSideExpressionis neither anObjectLiteralnor anArrayLiteral, then
    1. Let lref be the result of evaluatingLeftHandSideExpression.
    2. ReturnIfAbrupt(lref).
    3. IfIsAnonymousFunctionDefinition(AssignmentExpression) andIsIdentifierRefofLeftHandSideExpressionare bothtrue, then
      1. Let rval beNamedEvaluationofAssignmentExpressionwith argument lref.[[ReferencedName]].
    4. Else,
      1. Let rref be the result of evaluatingAssignmentExpression.
      2. Let rval be ? GetValue(rref).
    5. Perform ? PutValue(lref, rval).
    6. Return rval.
  2. Let assignmentPattern be theAssignmentPatternthat iscoveredbyLeftHandSideExpression.
  3. Let rref be the result of evaluatingAssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Perform ?DestructuringAssignmentEvaluationof assignmentPattern using rval as the argument.
  6. Return rval.
AssignmentExpression:LeftHandSideExpressionAssignmentOperatorAssignmentExpression
  1. Let lref be the result of evaluatingLeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluatingAssignmentExpression.
  4. Let rval be ? GetValue(rref).
  5. Let assignmentOpText be the source text matched byAssignmentOperator.
  6. Let opText be the sequence of Unicode code points associated with assignmentOpText in the following table:
    assignmentOpTextopText
    **=**
    *=*
    /=/
    %=%
    +=+
    -=-
    <<=<<
    >>=>>
    >>>=>>>
    &=&
    ^=^
    |=|
  7. Let r beApplyStringOrNumericBinaryOperator(lval, opText, rval).
  8. Perform ? PutValue(lref, r).
  9. Return r.
AssignmentExpression:LeftHandSideExpression&&=AssignmentExpression
  1. Let lref be the result of evaluatingLeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool isfalse, return lval.
  5. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrueandIsIdentifierRefofLeftHandSideExpressionistrue, then
    1. Let rval beNamedEvaluationofAssignmentExpressionwith argument lref.[[ReferencedName]].
  6. Else,
    1. Let rref be the result of evaluatingAssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
AssignmentExpression:LeftHandSideExpression||=AssignmentExpression
  1. Let lref be the result of evaluatingLeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. Let lbool be ! ToBoolean(lval).
  4. If lbool istrue, return lval.
  5. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrueandIsIdentifierRefofLeftHandSideExpressionistrue, then
    1. Let rval beNamedEvaluationofAssignmentExpressionwith argument lref.[[ReferencedName]].
  6. Else,
    1. Let rref be the result of evaluatingAssignmentExpression.
    2. Let rval be ? GetValue(rref).
  7. Perform ? PutValue(lref, rval).
  8. Return rval.
AssignmentExpression:LeftHandSideExpression??=AssignmentExpression
  1. Let lref be the result of evaluatingLeftHandSideExpression.
  2. Let lval be ? GetValue(lref).
  3. If lval is neitherundefinednornull, return lval.
  4. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrueandIsIdentifierRefofLeftHandSideExpressionistrue, then
    1. Let rval beNamedEvaluationofAssignmentExpressionwith argument lref.[[ReferencedName]].
  5. Else,
    1. Let rref be the result of evaluatingAssignmentExpression.
    2. Let rval be ? GetValue(rref).
  6. Perform ? PutValue(lref, rval).
  7. Return rval.
Note

When this expression occurs withinstrict mode code, it is a runtime error if lref in step1.e,2,2,2,2is an unresolvable reference. If it is, aReferenceErrorexception is thrown. Additionally, it is a runtime error if the lref in step8,7,7,6is a reference to adata propertywith the attribute value { [[Writable]]:false}, to anaccessor propertywith the attribute value { [[Set]]:undefined}, or to a non-existent property of an object for which theIsExtensiblepredicate returns the valuefalse. In these cases aTypeErrorexception is thrown.

13.15.3 ApplyStringOrNumericBinaryOperator ( lval, opText, rval )

The abstract operation ApplyStringOrNumericBinaryOperator takes arguments lval (anECMAScript language value), opText (a sequence of Unicode code points), and rval (anECMAScript language value). It performs the following steps when called:

  1. Assert: opText is present in the table in step8.
  2. If opText is +, then
    1. Let lprim be ? ToPrimitive(lval).
    2. Let rprim be ? ToPrimitive(rval).
    3. IfType(lprim) is String orType(rprim) is String, then
      1. Let lstr be ? ToString(lprim).
      2. Let rstr be ? ToString(rprim).
      3. Return thestring-concatenationof lstr and rstr.
    4. Set lval to lprim.
    5. Set rval to rprim.
  3. NOTE: At this point, it must be a numeric operation.
  4. Let lnum be ? ToNumeric(lval).
  5. Let rnum be ? ToNumeric(rval).
  6. IfType(lnum) is different fromType(rnum), throw aTypeErrorexception.
  7. Let T beType(lnum).
  8. Let operation be the abstract operation associated with opText in the following table:
    opTextoperation
    **T::exponentiate
    *T::multiply
    /T::divide
    %T::remainder
    +T::add
    -T::subtract
    <<T::leftShift
    >>T::signedRightShift
    >>>T::unsignedRightShift
    &T::bitwiseAND
    ^T::bitwiseXOR
    |T::bitwiseOR
  9. Return ? operation(lnum, rnum).
Note 1

No hint is provided in the calls toToPrimitivein steps2.aand2.b. All standard objects except Date objects handle the absence of a hint as ifnumberwere given; Date objects handle the absence of a hint as ifstringwere given. Exotic objects may handle the absence of a hint in some other manner.

Note 2

Step2.cdiffers from step3of theIsLessThanalgorithm, by using the logical-or operation instead of the logical-and operation.

13.15.4 EvaluateStringOrNumericBinaryExpression ( leftOperand, opText, rightOperand )

The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (aParse Node), opText (a sequence of Unicode code points), and rightOperand (aParse Node). It performs the following steps when called:

  1. Let lref be the result of evaluating leftOperand.
  2. Let lval be ? GetValue(lref).
  3. Let rref be the result of evaluating rightOperand.
  4. Let rval be ? GetValue(rref).
  5. Return ? ApplyStringOrNumericBinaryOperator(lval, opText, rval).

13.15.5 Destructuring Assignment

Supplemental Syntax

In certain circumstances when processing an instance of the production
AssignmentExpression:LeftHandSideExpression=AssignmentExpression
the interpretation ofLeftHandSideExpressionis refined using the following grammar:

AssignmentPattern[Yield, Await]:ObjectAssignmentPattern[?Yield, ?Await]ArrayAssignmentPattern[?Yield, ?Await]ObjectAssignmentPattern[Yield, Await]:{}{AssignmentRestProperty[?Yield, ?Await]}{AssignmentPropertyList[?Yield, ?Await]}{AssignmentPropertyList[?Yield, ?Await],AssignmentRestProperty[?Yield, ?Await]opt}ArrayAssignmentPattern[Yield, Await]:[ElisionoptAssignmentRestElement[?Yield, ?Await]opt][AssignmentElementList[?Yield, ?Await]][AssignmentElementList[?Yield, ?Await],ElisionoptAssignmentRestElement[?Yield, ?Await]opt]AssignmentRestProperty[Yield, Await]:...DestructuringAssignmentTarget[?Yield, ?Await]AssignmentPropertyList[Yield, Await]:AssignmentProperty[?Yield, ?Await]AssignmentPropertyList[?Yield, ?Await],AssignmentProperty[?Yield, ?Await]AssignmentElementList[Yield, Await]:AssignmentElisionElement[?Yield, ?Await]AssignmentElementList[?Yield, ?Await],AssignmentElisionElement[?Yield, ?Await]AssignmentElisionElement[Yield, Await]:ElisionoptAssignmentElement[?Yield, ?Await]AssignmentProperty[Yield, Await]:IdentifierReference[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optPropertyName[?Yield, ?Await]:AssignmentElement[?Yield, ?Await]AssignmentElement[Yield, Await]:DestructuringAssignmentTarget[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optAssignmentRestElement[Yield, Await]:...DestructuringAssignmentTarget[?Yield, ?Await]DestructuringAssignmentTarget[Yield, Await]:LeftHandSideExpression[?Yield, ?Await]

13.15.5.1 Static Semantics: Early Errors

AssignmentProperty:IdentifierReferenceInitializeroptAssignmentRestProperty:...DestructuringAssignmentTargetDestructuringAssignmentTarget:LeftHandSideExpression

IfLeftHandSideExpressionis anObjectLiteralor anArrayLiteral, the following Early Error rules are applied:

IfLeftHandSideExpressionis neither anObjectLiteralnor anArrayLiteral, the following Early Error rule is applied:

13.15.5.2 Runtime Semantics: DestructuringAssignmentEvaluation

With parameter value.

ObjectAssignmentPattern:{}
  1. Perform ? RequireObjectCoercible(value).
  2. ReturnNormalCompletion(empty).
ObjectAssignmentPattern:{AssignmentPropertyList}{AssignmentPropertyList,}
  1. Perform ? RequireObjectCoercible(value).
  2. Perform ?PropertyDestructuringAssignmentEvaluationforAssignmentPropertyListusing value as the argument.
  3. ReturnNormalCompletion(empty).
ArrayAssignmentPattern:[]
  1. Let iteratorRecord be ? GetIterator(value).
  2. Return ? IteratorClose(iteratorRecord,NormalCompletion(empty)).
ArrayAssignmentPattern:[Elision]
  1. Let iteratorRecord be ? GetIterator(value).
  2. Let result beIteratorDestructuringAssignmentEvaluationofElisionwith argument iteratorRecord.
  3. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, result).
  4. Return result.
ArrayAssignmentPattern:[ElisionoptAssignmentRestElement]
  1. Let iteratorRecord be ? GetIterator(value).
  2. IfElisionis present, then
    1. Let status beIteratorDestructuringAssignmentEvaluationofElisionwith argument iteratorRecord.
    2. If status is anabrupt completion, then
      1. Assert: iteratorRecord.[[Done]] istrue.
      2. ReturnCompletion(status).
  3. Let result beIteratorDestructuringAssignmentEvaluationofAssignmentRestElementwith argument iteratorRecord.
  4. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, result).
  5. Return result.
ArrayAssignmentPattern:[AssignmentElementList]
  1. Let iteratorRecord be ? GetIterator(value).
  2. Let result beIteratorDestructuringAssignmentEvaluationofAssignmentElementListwith argument iteratorRecord.
  3. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, result).
  4. Return result.
ArrayAssignmentPattern:[AssignmentElementList,ElisionoptAssignmentRestElementopt]
  1. Let iteratorRecord be ? GetIterator(value).
  2. Let status beIteratorDestructuringAssignmentEvaluationofAssignmentElementListwith argument iteratorRecord.
  3. If status is anabrupt completion, then
    1. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, status).
    2. ReturnCompletion(status).
  4. IfElisionis present, then
    1. Set status to the result of performingIteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
    2. If status is anabrupt completion, then
      1. Assert: iteratorRecord.[[Done]] istrue.
      2. ReturnCompletion(status).
  5. IfAssignmentRestElementis present, then
    1. Set status to the result of performingIteratorDestructuringAssignmentEvaluationofAssignmentRestElementwith iteratorRecord as the argument.
  6. If iteratorRecord.[[Done]] isfalse, return ? IteratorClose(iteratorRecord, status).
  7. ReturnCompletion(status).
ObjectAssignmentPattern:{AssignmentRestProperty}
  1. Perform ? RequireObjectCoercible(value).
  2. Let excludedNames be a new emptyList.
  3. Return the result of performingRestDestructuringAssignmentEvaluationofAssignmentRestPropertywith value and excludedNames as the arguments.
ObjectAssignmentPattern:{AssignmentPropertyList,AssignmentRestProperty}
  1. Perform ? RequireObjectCoercible(value).
  2. Let excludedNames be ?PropertyDestructuringAssignmentEvaluationofAssignmentPropertyListwith argument value.
  3. Return the result of performingRestDestructuringAssignmentEvaluationofAssignmentRestPropertywith arguments value and excludedNames.

13.15.5.3 Runtime Semantics: PropertyDestructuringAssignmentEvaluation

With parameter value.

Note
The following operations collect a list of all destructured property names.
AssignmentPropertyList:AssignmentPropertyList,AssignmentProperty
  1. Let propertyNames be ?PropertyDestructuringAssignmentEvaluationofAssignmentPropertyListwith argument value.
  2. Let nextNames be ?PropertyDestructuringAssignmentEvaluationofAssignmentPropertywith argument value.
  3. Return thelist-concatenationof propertyNames and nextNames.
AssignmentProperty:IdentifierReferenceInitializeropt
  1. Let P beStringValueofIdentifierReference.
  2. Let lref be ? ResolveBinding(P).
  3. Let v be ? GetV(value, P).
  4. IfInitializeroptis present and v isundefined, then
    1. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
      1. Set v to the result of performingNamedEvaluationforInitializerwith argument P.
    2. Else,
      1. Let defaultValue be the result of evaluatingInitializer.
      2. Set v to ? GetValue(defaultValue).
  5. Perform ? PutValue(lref, v).
  6. Return aListwhose sole element is P.
AssignmentProperty:PropertyName:AssignmentElement
  1. Let name be the result of evaluatingPropertyName.
  2. ReturnIfAbrupt(name).
  3. Perform ?KeyedDestructuringAssignmentEvaluationofAssignmentElementwith value and name as the arguments.
  4. Return aListwhose sole element is name.

13.15.5.4 Runtime Semantics: RestDestructuringAssignmentEvaluation

With parameters value and excludedNames.

AssignmentRestProperty:...DestructuringAssignmentTarget
  1. Let lref be the result of evaluatingDestructuringAssignmentTarget.
  2. ReturnIfAbrupt(lref).
  3. Let restObj be ! OrdinaryObjectCreate(%Object.prototype%).
  4. Perform ? CopyDataProperties(restObj, value, excludedNames).
  5. ReturnPutValue(lref, restObj).

13.15.5.5 Runtime Semantics: IteratorDestructuringAssignmentEvaluation

With parameter iteratorRecord.

AssignmentElementList:AssignmentElisionElement
  1. Return the result of performingIteratorDestructuringAssignmentEvaluationofAssignmentElisionElementusing iteratorRecord as the argument.
AssignmentElementList:AssignmentElementList,AssignmentElisionElement
  1. Perform ?IteratorDestructuringAssignmentEvaluationofAssignmentElementListusing iteratorRecord as the argument.
  2. Return the result of performingIteratorDestructuringAssignmentEvaluationofAssignmentElisionElementusing iteratorRecord as the argument.
AssignmentElisionElement:AssignmentElement
  1. Return the result of performingIteratorDestructuringAssignmentEvaluationofAssignmentElementwith iteratorRecord as the argument.
AssignmentElisionElement:ElisionAssignmentElement
  1. Perform ?IteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
  2. Return the result of performingIteratorDestructuringAssignmentEvaluationofAssignmentElementwith iteratorRecord as the argument.
Elision:,
  1. If iteratorRecord.[[Done]] isfalse, then
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
  2. ReturnNormalCompletion(empty).
Elision:Elision,
  1. Perform ?IteratorDestructuringAssignmentEvaluationofElisionwith iteratorRecord as the argument.
  2. If iteratorRecord.[[Done]] isfalse, then
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
  3. ReturnNormalCompletion(empty).
AssignmentElement:DestructuringAssignmentTargetInitializeropt
  1. IfDestructuringAssignmentTargetis neither anObjectLiteralnor anArrayLiteral, then
    1. Let lref be the result of evaluatingDestructuringAssignmentTarget.
    2. ReturnIfAbrupt(lref).
  2. If iteratorRecord.[[Done]] isfalse, then
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    5. Else,
      1. Let value beIteratorValue(next).
      2. If value is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(value).
  3. If iteratorRecord.[[Done]] istrue, let value beundefined.
  4. IfInitializeris present and value isundefined, then
    1. IfIsAnonymousFunctionDefinition(Initializer) istrueandIsIdentifierRefofDestructuringAssignmentTargetistrue, then
      1. Let v be ?NamedEvaluationofInitializerwith argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be the result of evaluatingInitializer.
      2. Let v be ? GetValue(defaultValue).
  5. Else, let v be value.
  6. IfDestructuringAssignmentTargetis anObjectLiteralor anArrayLiteral, then
    1. Let nestedAssignmentPattern be theAssignmentPatternthat iscoveredbyDestructuringAssignmentTarget.
    2. Return the result of performingDestructuringAssignmentEvaluationof nestedAssignmentPattern with v as the argument.
  7. Return ? PutValue(lref, v).
Note

Left to right evaluation order is maintained by evaluating aDestructuringAssignmentTargetthat is not a destructuring pattern prior to accessing the iterator or evaluating theInitializer.

AssignmentRestElement:...DestructuringAssignmentTarget
  1. IfDestructuringAssignmentTargetis neither anObjectLiteralnor anArrayLiteral, then
    1. Let lref be the result of evaluatingDestructuringAssignmentTarget.
    2. ReturnIfAbrupt(lref).
  2. Let A be ! ArrayCreate(0).
  3. Let n be 0.
  4. Repeat, while iteratorRecord.[[Done]] isfalse,
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, set iteratorRecord.[[Done]] totrue.
    5. Else,
      1. Let nextValue beIteratorValue(next).
      2. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
      3. ReturnIfAbrupt(nextValue).
      4. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), nextValue).
      5. Set n to n + 1.
  5. IfDestructuringAssignmentTargetis neither anObjectLiteralnor anArrayLiteral, then
    1. Return ? PutValue(lref, A).
  6. Let nestedAssignmentPattern be theAssignmentPatternthat iscoveredbyDestructuringAssignmentTarget.
  7. Return the result of performingDestructuringAssignmentEvaluationof nestedAssignmentPattern with A as the argument.

13.15.5.6 Runtime Semantics: KeyedDestructuringAssignmentEvaluation

With parameters value and propertyName.

AssignmentElement:DestructuringAssignmentTargetInitializeropt
  1. IfDestructuringAssignmentTargetis neither anObjectLiteralnor anArrayLiteral, then
    1. Let lref be the result of evaluatingDestructuringAssignmentTarget.
    2. ReturnIfAbrupt(lref).
  2. Let v be ? GetV(value, propertyName).
  3. IfInitializeris present and v isundefined, then
    1. IfIsAnonymousFunctionDefinition(Initializer) andIsIdentifierRefofDestructuringAssignmentTargetare bothtrue, then
      1. Let rhsValue be ?NamedEvaluationofInitializerwith argument lref.[[ReferencedName]].
    2. Else,
      1. Let defaultValue be the result of evaluatingInitializer.
      2. Let rhsValue be ? GetValue(defaultValue).
  4. Else, let rhsValue be v.
  5. IfDestructuringAssignmentTargetis anObjectLiteralor anArrayLiteral, then
    1. Let assignmentPattern be theAssignmentPatternthat iscoveredbyDestructuringAssignmentTarget.
    2. Return the result of performingDestructuringAssignmentEvaluationof assignmentPattern with rhsValue as the argument.
  6. Return ? PutValue(lref, rhsValue).

13.16 Comma Operator ( , )

Syntax

Expression[In, Yield, Await]:AssignmentExpression[?In, ?Yield, ?Await]Expression[?In, ?Yield, ?Await],AssignmentExpression[?In, ?Yield, ?Await]

13.16.1 Runtime Semantics: Evaluation

Expression:Expression,AssignmentExpression
  1. Let lref be the result of evaluatingExpression.
  2. Perform ? GetValue(lref).
  3. Let rref be the result of evaluatingAssignmentExpression.
  4. Return ? GetValue(rref).
Note

GetValuemust be called even though its value is not used because it may have observable side-effects.

14 ECMAScript Language: Statements and Declarations

Syntax

Statement[Yield, Await, Return]:BlockStatement[?Yield, ?Await, ?Return]VariableStatement[?Yield, ?Await]EmptyStatementExpressionStatement[?Yield, ?Await]IfStatement[?Yield, ?Await, ?Return]BreakableStatement[?Yield, ?Await, ?Return]ContinueStatement[?Yield, ?Await]BreakStatement[?Yield, ?Await][+Return]ReturnStatement[?Yield, ?Await]WithStatement[?Yield, ?Await, ?Return]LabelledStatement[?Yield, ?Await, ?Return]ThrowStatement[?Yield, ?Await]TryStatement[?Yield, ?Await, ?Return]DebuggerStatementDeclaration[Yield, Await]:HoistableDeclaration[?Yield, ?Await, ~Default]ClassDeclaration[?Yield, ?Await, ~Default]LexicalDeclaration[+In, ?Yield, ?Await]HoistableDeclaration[Yield, Await, Default]:FunctionDeclaration[?Yield, ?Await, ?Default]GeneratorDeclaration[?Yield, ?Await, ?Default]AsyncFunctionDeclaration[?Yield, ?Await, ?Default]AsyncGeneratorDeclaration[?Yield, ?Await, ?Default]BreakableStatement[Yield, Await, Return]:IterationStatement[?Yield, ?Await, ?Return]SwitchStatement[?Yield, ?Await, ?Return]

14.1 Statement Semantics

14.1.1 Runtime Semantics: Evaluation

HoistableDeclaration:GeneratorDeclarationAsyncFunctionDeclarationAsyncGeneratorDeclaration
  1. ReturnNormalCompletion(empty).
HoistableDeclaration:FunctionDeclaration
  1. Return the result of evaluatingFunctionDeclaration.
BreakableStatement:IterationStatementSwitchStatement
  1. Let newLabelSet be a new emptyList.
  2. Return the result of performingLabelledEvaluationof thisBreakableStatementwith argument newLabelSet.

14.2 Block

Syntax

BlockStatement[Yield, Await, Return]:Block[?Yield, ?Await, ?Return]Block[Yield, Await, Return]:{StatementList[?Yield, ?Await, ?Return]opt}StatementList[Yield, Await, Return]:StatementListItem[?Yield, ?Await, ?Return]StatementList[?Yield, ?Await, ?Return]StatementListItem[?Yield, ?Await, ?Return]StatementListItem[Yield, Await, Return]:Statement[?Yield, ?Await, ?Return]Declaration[?Yield, ?Await]

14.2.1 Static Semantics: Early Errors

Block:{StatementList}

14.2.2 Runtime Semantics: Evaluation

Block:{}
  1. ReturnNormalCompletion(empty).
Block:{StatementList}
  1. Let oldEnv be therunning execution context's LexicalEnvironment.
  2. Let blockEnv beNewDeclarativeEnvironment(oldEnv).
  3. PerformBlockDeclarationInstantiation(StatementList, blockEnv).
  4. Set therunning execution context's LexicalEnvironment to blockEnv.
  5. Let blockValue be the result of evaluatingStatementList.
  6. Set therunning execution context's LexicalEnvironment to oldEnv.
  7. Return blockValue.
Note 1

No matter how control leaves theBlockthe LexicalEnvironment is always restored to its former state.

StatementList:StatementListStatementListItem
  1. Let sl be the result of evaluatingStatementList.
  2. ReturnIfAbrupt(sl).
  3. Let s be the result of evaluatingStatementListItem.
  4. ReturnCompletion(UpdateEmpty(s, sl)).
Note 2

The value of aStatementListis the value of the last value-producing item in theStatementList. For example, the following calls to the eval function all return the value 1:

eval("1;;;;;")
eval("1;{}")
eval("1;var a;")

14.2.3 BlockDeclarationInstantiation ( code, env )

The abstract operation BlockDeclarationInstantiation takes arguments code (aParse Node) and env (anEnvironment Record). code is theParse Nodecorresponding to the body of the block. env is theEnvironment Recordin which bindings are to be created.

Note

When aBlockorCaseBlockis evaluated a newdeclarative Environment Recordis created and bindings for each block scoped variable, constant, function, or class declared in the block are instantiated in theEnvironment Record.

It performs the following steps when called:

  1. Assert: env is adeclarative Environment Record.
  2. Let declarations be theLexicallyScopedDeclarationsof code.
  3. Let privateEnv be therunning execution context's PrivateEnvironment.
  4. For each element d of declarations, do
    1. For each element dn of theBoundNamesof d, do
      1. IfIsConstantDeclarationof d istrue, then
        1. Perform ! env.CreateImmutableBinding(dn,true).
      2. Else,
        1. Perform ! env.CreateMutableBinding(dn,false). NOTE: This step is replaced in sectionB.3.2.6.
    2. If d is aFunctionDeclaration, aGeneratorDeclaration, anAsyncFunctionDeclaration, or anAsyncGeneratorDeclaration, then
      1. Let fn be the sole element of theBoundNamesof d.
      2. Let fo beInstantiateFunctionObjectof d with arguments env and privateEnv.
      3. Perform env.InitializeBinding(fn, fo). NOTE: This step is replaced in sectionB.3.2.6.

14.3 Declarations and the Variable Statement

14.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to therunning execution context's LexicalEnvironment. The variables are created when their containingEnvironment Recordis instantiated but may not be accessed in any way until the variable'sLexicalBindingis evaluated. A variable defined by aLexicalBindingwith anInitializeris assigned the value of itsInitializer'sAssignmentExpressionwhen theLexicalBindingis evaluated, not when the variable is created. If aLexicalBindingin a let declaration does not have anInitializerthe variable is assigned the valueundefinedwhen theLexicalBindingis evaluated.

Syntax

LexicalDeclaration[In, Yield, Await]:LetOrConstBindingList[?In, ?Yield, ?Await];LetOrConst:letconstBindingList[In, Yield, Await]:LexicalBinding[?In, ?Yield, ?Await]BindingList[?In, ?Yield, ?Await],LexicalBinding[?In, ?Yield, ?Await]LexicalBinding[In, Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]optBindingPattern[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]

14.3.1.1 Static Semantics: Early Errors

LexicalDeclaration:LetOrConstBindingList;LexicalBinding:BindingIdentifierInitializeropt

14.3.1.2 Runtime Semantics: Evaluation

LexicalDeclaration:LetOrConstBindingList;
  1. Let next be the result of evaluatingBindingList.
  2. ReturnIfAbrupt(next).
  3. ReturnNormalCompletion(empty).
BindingList:BindingList,LexicalBinding
  1. Let next be the result of evaluatingBindingList.
  2. ReturnIfAbrupt(next).
  3. Return the result of evaluatingLexicalBinding.
LexicalBinding:BindingIdentifier
  1. Let lhs beResolveBinding(StringValueofBindingIdentifier).
  2. ReturnInitializeReferencedBinding(lhs,undefined).
Note

Astatic semanticsrule ensures that this form ofLexicalBindingnever occurs in a const declaration.

LexicalBinding:BindingIdentifierInitializer
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Let lhs beResolveBinding(bindingId).
  3. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
    1. Let value beNamedEvaluationofInitializerwith argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluatingInitializer.
    2. Let value be ? GetValue(rhs).
  5. ReturnInitializeReferencedBinding(lhs, value).
LexicalBinding:BindingPatternInitializer
  1. Let rhs be the result of evaluatingInitializer.
  2. Let value be ? GetValue(rhs).
  3. Let env be therunning execution context's LexicalEnvironment.
  4. Return the result of performingBindingInitializationforBindingPatternusing value and env as the arguments.

14.3.2 Variable Statement

Note

A var statement declares variables that are scoped to therunning execution context's VariableEnvironment. Var variables are created when their containingEnvironment Recordis instantiated and are initialized toundefinedwhen created. Within the scope of any VariableEnvironment a commonBindingIdentifiermay appear in more than oneVariableDeclarationbut those declarations collectively define only one variable. A variable defined by aVariableDeclarationwith anInitializeris assigned the value of itsInitializer'sAssignmentExpressionwhen theVariableDeclarationis executed, not when the variable is created.

Syntax

VariableStatement[Yield, Await]:varVariableDeclarationList[+In, ?Yield, ?Await];VariableDeclarationList[In, Yield, Await]:VariableDeclaration[?In, ?Yield, ?Await]VariableDeclarationList[?In, ?Yield, ?Await],VariableDeclaration[?In, ?Yield, ?Await]VariableDeclaration[In, Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]optBindingPattern[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]

14.3.2.1 Runtime Semantics: Evaluation

VariableStatement:varVariableDeclarationList;
  1. Let next be the result of evaluatingVariableDeclarationList.
  2. ReturnIfAbrupt(next).
  3. ReturnNormalCompletion(empty).
VariableDeclarationList:VariableDeclarationList,VariableDeclaration
  1. Let next be the result of evaluatingVariableDeclarationList.
  2. ReturnIfAbrupt(next).
  3. Return the result of evaluatingVariableDeclaration.
VariableDeclaration:BindingIdentifier
  1. ReturnNormalCompletion(empty).
VariableDeclaration:BindingIdentifierInitializer
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
    1. Let value beNamedEvaluationofInitializerwith argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluatingInitializer.
    2. Let value be ? GetValue(rhs).
  5. Return ? PutValue(lhs, value).
Note

If aVariableDeclarationis nested within a with statement and theBindingIdentifierin theVariableDeclarationis the same as aproperty nameof the binding object of the with statement'sobject Environment Record, then step5will assign value to the property instead of assigning to the VariableEnvironment binding of theIdentifier.

VariableDeclaration:BindingPatternInitializer
  1. Let rhs be the result of evaluatingInitializer.
  2. Let rval be ? GetValue(rhs).
  3. Return the result of performingBindingInitializationforBindingPatternpassing rval andundefinedas arguments.

14.3.3 Destructuring Binding Patterns

Syntax

BindingPattern[Yield, Await]:ObjectBindingPattern[?Yield, ?Await]ArrayBindingPattern[?Yield, ?Await]ObjectBindingPattern[Yield, Await]:{}{BindingRestProperty[?Yield, ?Await]}{BindingPropertyList[?Yield, ?Await]}{BindingPropertyList[?Yield, ?Await],BindingRestProperty[?Yield, ?Await]opt}ArrayBindingPattern[Yield, Await]:[ElisionoptBindingRestElement[?Yield, ?Await]opt][BindingElementList[?Yield, ?Await]][BindingElementList[?Yield, ?Await],ElisionoptBindingRestElement[?Yield, ?Await]opt]BindingRestProperty[Yield, Await]:...BindingIdentifier[?Yield, ?Await]BindingPropertyList[Yield, Await]:BindingProperty[?Yield, ?Await]BindingPropertyList[?Yield, ?Await],BindingProperty[?Yield, ?Await]BindingElementList[Yield, Await]:BindingElisionElement[?Yield, ?Await]BindingElementList[?Yield, ?Await],BindingElisionElement[?Yield, ?Await]BindingElisionElement[Yield, Await]:ElisionoptBindingElement[?Yield, ?Await]BindingProperty[Yield, Await]:SingleNameBinding[?Yield, ?Await]PropertyName[?Yield, ?Await]:BindingElement[?Yield, ?Await]BindingElement[Yield, Await]:SingleNameBinding[?Yield, ?Await]BindingPattern[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optSingleNameBinding[Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optBindingRestElement[Yield, Await]:...BindingIdentifier[?Yield, ?Await]...BindingPattern[?Yield, ?Await]

14.3.3.1 Runtime Semantics: PropertyBindingInitialization

With parameters value and environment.

Note
These collect a list of all bound property names rather than just empty completion.
BindingPropertyList:BindingPropertyList,BindingProperty
  1. Let boundNames be ?PropertyBindingInitializationofBindingPropertyListwith arguments value and environment.
  2. Let nextNames be ?PropertyBindingInitializationofBindingPropertywith arguments value and environment.
  3. Return thelist-concatenationof boundNames and nextNames.
BindingProperty:SingleNameBinding
  1. Let name be the string that is the only element ofBoundNamesofSingleNameBinding.
  2. Perform ?KeyedBindingInitializationforSingleNameBindingusing value, environment, and name as the arguments.
  3. Return aListwhose sole element is name.
BindingProperty:PropertyName:BindingElement
  1. Let P be the result of evaluatingPropertyName.
  2. ReturnIfAbrupt(P).
  3. Perform ?KeyedBindingInitializationofBindingElementwith value, environment, and P as the arguments.
  4. Return aListwhose sole element is P.

14.3.3.2 Runtime Semantics: RestBindingInitialization

With parameters value, environment, and excludedNames.

BindingRestProperty:...BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValueofBindingIdentifier, environment).
  2. Let restObj be ! OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ? CopyDataProperties(restObj, value, excludedNames).
  4. If environment isundefined, returnPutValue(lhs, restObj).
  5. ReturnInitializeReferencedBinding(lhs, restObj).

14.3.3.3 Runtime Semantics: KeyedBindingInitialization

With parameters value, environment, and propertyName.

Note

Whenundefinedis passed for environment it indicates that aPutValueoperation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

BindingElement:BindingPatternInitializeropt
  1. Let v be ? GetV(value, propertyName).
  2. IfInitializeris present and v isundefined, then
    1. Let defaultValue be the result of evaluatingInitializer.
    2. Set v to ? GetValue(defaultValue).
  3. Return the result of performingBindingInitializationforBindingPatternpassing v and environment as arguments.
SingleNameBinding:BindingIdentifierInitializeropt
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be ? GetV(value, propertyName).
  4. IfInitializeris present and v isundefined, then
    1. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
      1. Set v to the result of performingNamedEvaluationforInitializerwith argument bindingId.
    2. Else,
      1. Let defaultValue be the result of evaluatingInitializer.
      2. Set v to ? GetValue(defaultValue).
  5. If environment isundefined, return ? PutValue(lhs, v).
  6. ReturnInitializeReferencedBinding(lhs, v).

14.4 Empty Statement

Syntax

EmptyStatement:;

14.4.1 Runtime Semantics: Evaluation

EmptyStatement:;
  1. ReturnNormalCompletion(empty).

14.5 Expression Statement

Syntax

ExpressionStatement[Yield, Await]:[lookahead ∉ {{,function,async[noLineTerminatorhere]function,class,let[}]Expression[+In, ?Yield, ?Await];Note

AnExpressionStatementcannot start with a U+007B (LEFT CURLY BRACKET) because that might make it ambiguous with aBlock. AnExpressionStatementcannot start with the function or class keywords because that would make it ambiguous with aFunctionDeclaration, aGeneratorDeclaration, or aClassDeclaration. AnExpressionStatementcannot start with async function because that would make it ambiguous with anAsyncFunctionDeclarationor aAsyncGeneratorDeclaration. AnExpressionStatementcannot start with the two token sequence let [ because that would make it ambiguous with a letLexicalDeclarationwhose firstLexicalBindingwas anArrayBindingPattern.

14.5.1 Runtime Semantics: Evaluation

ExpressionStatement:Expression;
  1. Let exprRef be the result of evaluatingExpression.
  2. Return ? GetValue(exprRef).

14.6 The if Statement

Syntax

IfStatement[Yield, Await, Return]:if(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]elseStatement[?Yield, ?Await, ?Return]if(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][lookahead ≠else]Note
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs

14.6.1 Static Semantics: Early Errors

IfStatement:if(Expression)StatementelseStatementIfStatement:if(Expression)StatementNote

It is only necessary to apply this rule if the extension specified inB.3.1is implemented.

14.6.2 Runtime Semantics: Evaluation

IfStatement:if(Expression)StatementelseStatement
  1. Let exprRef be the result of evaluatingExpression.
  2. Let exprValue be ! ToBoolean(?GetValue(exprRef)).
  3. If exprValue istrue, then
    1. Let stmtCompletion be the result of evaluating the firstStatement.
  4. Else,
    1. Let stmtCompletion be the result of evaluating the secondStatement.
  5. ReturnCompletion(UpdateEmpty(stmtCompletion,undefined)).
IfStatement:if(Expression)Statement
  1. Let exprRef be the result of evaluatingExpression.
  2. Let exprValue be ! ToBoolean(?GetValue(exprRef)).
  3. If exprValue isfalse, then
    1. ReturnNormalCompletion(undefined).
  4. Else,
    1. Let stmtCompletion be the result of evaluatingStatement.
    2. ReturnCompletion(UpdateEmpty(stmtCompletion,undefined)).

14.7 Iteration Statements

Syntax

IterationStatement[Yield, Await, Return]:DoWhileStatement[?Yield, ?Await, ?Return]WhileStatement[?Yield, ?Await, ?Return]ForStatement[?Yield, ?Await, ?Return]ForInOfStatement[?Yield, ?Await, ?Return]

14.7.1 Semantics

14.7.1.1 LoopContinues ( completion, labelSet )

The abstract operation LoopContinues takes arguments completion and labelSet. It performs the following steps when called:

  1. If completion.[[Type]] isnormal, returntrue.
  2. If completion.[[Type]] is notcontinue, returnfalse.
  3. If completion.[[Target]] isempty, returntrue.
  4. If completion.[[Target]] is an element of labelSet, returntrue.
  5. Returnfalse.
Note

Within theStatementpart of anIterationStatementaContinueStatementmay be used to begin a new iteration.

14.7.1.2 Runtime Semantics: LoopEvaluation

With parameter labelSet.

IterationStatement:DoWhileStatement
  1. Return ?DoWhileLoopEvaluationofDoWhileStatementwith argument labelSet.
IterationStatement:WhileStatement
  1. Return ?WhileLoopEvaluationofWhileStatementwith argument labelSet.
IterationStatement:ForStatement
  1. Return ?ForLoopEvaluationofForStatementwith argument labelSet.
IterationStatement:ForInOfStatement
  1. Return ?ForInOfLoopEvaluationofForInOfStatementwith argument labelSet.

14.7.2 The do-while Statement

Syntax

DoWhileStatement[Yield, Await, Return]:doStatement[?Yield, ?Await, ?Return]while(Expression[+In, ?Yield, ?Await]);

14.7.2.1 Static Semantics: Early Errors

DoWhileStatement:doStatementwhile(Expression);Note

It is only necessary to apply this rule if the extension specified inB.3.1is implemented.

14.7.2.2 Runtime Semantics: DoWhileLoopEvaluation

With parameter labelSet.

DoWhileStatement:doStatementwhile(Expression);
  1. Let V beundefined.
  2. Repeat,
    1. Let stmtResult be the result of evaluatingStatement.
    2. IfLoopContinues(stmtResult, labelSet) isfalse, returnCompletion(UpdateEmpty(stmtResult, V)).
    3. If stmtResult.[[Value]] is notempty, set V to stmtResult.[[Value]].
    4. Let exprRef be the result of evaluatingExpression.
    5. Let exprValue be ? GetValue(exprRef).
    6. If ! ToBoolean(exprValue) isfalse, returnNormalCompletion(V).

14.7.3 The while Statement

Syntax

WhileStatement[Yield, Await, Return]:while(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]

14.7.3.1 Static Semantics: Early Errors

WhileStatement:while(Expression)StatementNote

It is only necessary to apply this rule if the extension specified inB.3.1is implemented.

14.7.3.2 Runtime Semantics: WhileLoopEvaluation

With parameter labelSet.

WhileStatement:while(Expression)Statement
  1. Let V beundefined.
  2. Repeat,
    1. Let exprRef be the result of evaluatingExpression.
    2. Let exprValue be ? GetValue(exprRef).
    3. If ! ToBoolean(exprValue) isfalse, returnNormalCompletion(V).
    4. Let stmtResult be the result of evaluatingStatement.
    5. IfLoopContinues(stmtResult, labelSet) isfalse, returnCompletion(UpdateEmpty(stmtResult, V)).
    6. If stmtResult.[[Value]] is notempty, set V to stmtResult.[[Value]].

14.7.4 The for Statement

Syntax

ForStatement[Yield, Await, Return]:for([lookahead ≠let[]Expression[~In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]for(varVariableDeclarationList[~In, ?Yield, ?Await];Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]for(LexicalDeclaration[~In, ?Yield, ?Await]Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]

14.7.4.1 Static Semantics: Early Errors

ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statementfor(varVariableDeclarationList;Expressionopt;Expressionopt)Statementfor(LexicalDeclarationExpressionopt;Expressionopt)StatementNote

It is only necessary to apply this rule if the extension specified inB.3.1is implemented.

ForStatement:for(LexicalDeclarationExpressionopt;Expressionopt)Statement

14.7.4.2 Runtime Semantics: ForLoopEvaluation

With parameter labelSet.

ForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statement
  1. If the firstExpressionis present, then
    1. Let exprRef be the result of evaluating the firstExpression.
    2. Perform ? GetValue(exprRef).
  2. Return ? ForBodyEvaluation(the secondExpression, the thirdExpression,Statement, « », labelSet).
ForStatement:for(varVariableDeclarationList;Expressionopt;Expressionopt)Statement
  1. Let varDcl be the result of evaluatingVariableDeclarationList.
  2. ReturnIfAbrupt(varDcl).
  3. Return ? ForBodyEvaluation(the firstExpression, the secondExpression,Statement, « », labelSet).
ForStatement:for(LexicalDeclarationExpressionopt;Expressionopt)Statement
  1. Let oldEnv be therunning execution context's LexicalEnvironment.
  2. Let loopEnv beNewDeclarativeEnvironment(oldEnv).
  3. Let isConst beIsConstantDeclarationofLexicalDeclaration.
  4. Let boundNames be theBoundNamesofLexicalDeclaration.
  5. For each element dn of boundNames, do
    1. If isConst istrue, then
      1. Perform ! loopEnv.CreateImmutableBinding(dn,true).
    2. Else,
      1. Perform ! loopEnv.CreateMutableBinding(dn,false).
  6. Set therunning execution context's LexicalEnvironment to loopEnv.
  7. Let forDcl be the result of evaluatingLexicalDeclaration.
  8. If forDcl is anabrupt completion, then
    1. Set therunning execution context's LexicalEnvironment to oldEnv.
    2. ReturnCompletion(forDcl).
  9. If isConst isfalse, let perIterationLets be boundNames; otherwise let perIterationLets be « ».
  10. Let bodyResult beForBodyEvaluation(the firstExpression, the secondExpression,Statement, perIterationLets, labelSet).
  11. Set therunning execution context's LexicalEnvironment to oldEnv.
  12. ReturnCompletion(bodyResult).

14.7.4.3 ForBodyEvaluation ( test, increment, stmt, perIterationBindings, labelSet )

The abstract operation ForBodyEvaluation takes arguments test, increment, stmt, perIterationBindings, and labelSet. It performs the following steps when called:

  1. Let V beundefined.
  2. Perform ? CreatePerIterationEnvironment(perIterationBindings).
  3. Repeat,
    1. If test is not[empty], then
      1. Let testRef be the result of evaluating test.
      2. Let testValue be ? GetValue(testRef).
      3. If ! ToBoolean(testValue) isfalse, returnNormalCompletion(V).
    2. Let result be the result of evaluating stmt.
    3. IfLoopContinues(result, labelSet) isfalse, returnCompletion(UpdateEmpty(result, V)).
    4. If result.[[Value]] is notempty, set V to result.[[Value]].
    5. Perform ? CreatePerIterationEnvironment(perIterationBindings).
    6. If increment is not[empty], then
      1. Let incRef be the result of evaluating increment.
      2. Perform ? GetValue(incRef).

14.7.4.4 CreatePerIterationEnvironment ( perIterationBindings )

The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings. It performs the following steps when called:

  1. If perIterationBindings has any elements, then
    1. Let lastIterationEnv be therunning execution context's LexicalEnvironment.
    2. Let outer be lastIterationEnv.[[OuterEnv]].
    3. Assert: outer is notnull.
    4. Let thisIterationEnv beNewDeclarativeEnvironment(outer).
    5. For each element bn of perIterationBindings, do
      1. Perform ! thisIterationEnv.CreateMutableBinding(bn,false).
      2. Let lastValue be ? lastIterationEnv.GetBindingValue(bn,true).
      3. Perform thisIterationEnv.InitializeBinding(bn, lastValue).
    6. Set therunning execution context's LexicalEnvironment to thisIterationEnv.
  2. Returnundefined.

14.7.5 The for-in, for-of, and for-await-of Statements

Syntax

ForInOfStatement[Yield, Await, Return]:for([lookahead ≠let[]LeftHandSideExpression[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(varForBinding[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(ForDeclaration[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for([lookahead ∉ {let,asyncof}]LeftHandSideExpression[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(varForBinding[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(ForDeclaration[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait([lookahead ≠let]LeftHandSideExpression[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait(varForBinding[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait(ForDeclaration[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]ForDeclaration[Yield, Await]:LetOrConstForBinding[?Yield, ?Await]ForBinding[Yield, Await]:BindingIdentifier[?Yield, ?Await]BindingPattern[?Yield, ?Await]Note

This section is extended by AnnexB.3.5.

14.7.5.1 Static Semantics: Early Errors

ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(varForBindinginExpression)Statementfor(ForDeclarationinExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementfor(varForBindingofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)StatementNote

It is only necessary to apply this rule if the extension specified inB.3.1is implemented.

ForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(LeftHandSideExpressionofAssignmentExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statement

IfLeftHandSideExpressionis either anObjectLiteralor anArrayLiteral, the following Early Error rules are applied:

IfLeftHandSideExpressionis neither anObjectLiteralnor anArrayLiteral, the following Early Error rule is applied:

ForInOfStatement:for(ForDeclarationinExpression)Statementfor(ForDeclarationofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)Statement

14.7.5.2 Static Semantics: IsDestructuring

MemberExpression:PrimaryExpression
  1. IfPrimaryExpressionis either anObjectLiteralor anArrayLiteral, returntrue.
  2. Returnfalse.
MemberExpression:MemberExpression[Expression]MemberExpression.IdentifierNameMemberExpressionTemplateLiteralSuperPropertyMetaPropertynewMemberExpressionArgumentsMemberExpression.PrivateIdentifierNewExpression:newNewExpressionLeftHandSideExpression:CallExpressionOptionalExpression
  1. Returnfalse.
ForDeclaration:LetOrConstForBinding
  1. ReturnIsDestructuringofForBinding.
ForBinding:BindingIdentifier
  1. Returnfalse.
ForBinding:BindingPattern
  1. Returntrue.
Note

This section is extended by AnnexB.3.5.

14.7.5.3 Runtime Semantics: ForDeclarationBindingInitialization

With parameters value and environment.

Note

undefinedis passed for environment to indicate that aPutValueoperation should be used to assign the initialization value. This is the case for var statements and the formal parameter lists of some non-strict functions (see10.2.11). In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.

ForDeclaration:LetOrConstForBinding
  1. Return the result of performingBindingInitializationforForBindingpassing value and environment as the arguments.

14.7.5.4 Runtime Semantics: ForDeclarationBindingInstantiation

With parameter environment.

ForDeclaration:LetOrConstForBinding
  1. Assert: environment is adeclarative Environment Record.
  2. For each element name of theBoundNamesofForBinding, do
    1. IfIsConstantDeclarationofLetOrConstistrue, then
      1. Perform ! environment.CreateImmutableBinding(name,true).
    2. Else,
      1. Perform ! environment.CreateMutableBinding(name,false).

14.7.5.5 Runtime Semantics: ForInOfLoopEvaluation

With parameter labelSet.

ForInOfStatement:for(LeftHandSideExpressioninExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,Expression,enumerate).
  2. Return ?ForIn/OfBodyEvaluation(LeftHandSideExpression,Statement, keyResult,enumerate,assignment, labelSet).
ForInOfStatement:for(varForBindinginExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,Expression,enumerate).
  2. Return ?ForIn/OfBodyEvaluation(ForBinding,Statement, keyResult,enumerate,varBinding, labelSet).
ForInOfStatement:for(ForDeclarationinExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(BoundNamesofForDeclaration,Expression,enumerate).
  2. Return ?ForIn/OfBodyEvaluation(ForDeclaration,Statement, keyResult,enumerate,lexicalBinding, labelSet).
ForInOfStatement:for(LeftHandSideExpressionofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,AssignmentExpression,iterate).
  2. Return ?ForIn/OfBodyEvaluation(LeftHandSideExpression,Statement, keyResult,iterate,assignment, labelSet).
ForInOfStatement:for(varForBindingofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,AssignmentExpression,iterate).
  2. Return ?ForIn/OfBodyEvaluation(ForBinding,Statement, keyResult,iterate,varBinding, labelSet).
ForInOfStatement:for(ForDeclarationofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(BoundNamesofForDeclaration,AssignmentExpression,iterate).
  2. Return ?ForIn/OfBodyEvaluation(ForDeclaration,Statement, keyResult,iterate,lexicalBinding, labelSet).
ForInOfStatement:forawait(LeftHandSideExpressionofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,AssignmentExpression,async-iterate).
  2. Return ?ForIn/OfBodyEvaluation(LeftHandSideExpression,Statement, keyResult,iterate,assignment, labelSet,async).
ForInOfStatement:forawait(varForBindingofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(« »,AssignmentExpression,async-iterate).
  2. Return ?ForIn/OfBodyEvaluation(ForBinding,Statement, keyResult,iterate,varBinding, labelSet,async).
ForInOfStatement:forawait(ForDeclarationofAssignmentExpression)Statement
  1. Let keyResult be ?ForIn/OfHeadEvaluation(BoundNamesofForDeclaration,AssignmentExpression,async-iterate).
  2. Return ?ForIn/OfBodyEvaluation(ForDeclaration,Statement, keyResult,iterate,lexicalBinding, labelSet,async).
Note

This section is extended by AnnexB.3.5.

14.7.5.6 ForIn/OfHeadEvaluation ( uninitializedBoundNames, expr, iterationKind )

The abstract operation ForIn/OfHeadEvaluation takes arguments uninitializedBoundNames, expr, and iterationKind (eitherenumerate,iterate, orasync-iterate). It performs the following steps when called:

  1. Let oldEnv be therunning execution context's LexicalEnvironment.
  2. If uninitializedBoundNames is not an emptyList, then
    1. Assert: uninitializedBoundNames has no duplicate entries.
    2. Let newEnv beNewDeclarativeEnvironment(oldEnv).
    3. For each String name of uninitializedBoundNames, do
      1. Perform ! newEnv.CreateMutableBinding(name,false).
    4. Set therunning execution context's LexicalEnvironment to newEnv.
  3. Let exprRef be the result of evaluating expr.
  4. Set therunning execution context's LexicalEnvironment to oldEnv.
  5. Let exprValue be ? GetValue(exprRef).
  6. If iterationKind isenumerate, then
    1. If exprValue isundefinedornull, then
      1. ReturnCompletion{ [[Type]]:break, [[Value]]:empty, [[Target]]:empty}.
    2. Let obj be ! ToObject(exprValue).
    3. Let iterator be ? EnumerateObjectProperties(obj).
    4. Let nextMethod be ! GetV(iterator,"next").
    5. Return theRecord{ [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]:false}.
  7. Else,
    1. Assert: iterationKind isiterateorasync-iterate.
    2. If iterationKind isasync-iterate, let iteratorHint beasync.
    3. Else, let iteratorHint besync.
    4. Return ? GetIterator(exprValue, iteratorHint).

14.7.5.7 ForIn/OfBodyEvaluation ( lhs, stmt, iteratorRecord, iterationKind, lhsKind, labelSet [ , iteratorKind ] )

The abstract operation ForIn/OfBodyEvaluation takes arguments lhs, stmt, iteratorRecord, iterationKind, lhsKind (eitherassignment,varBindingorlexicalBinding), and labelSet and optional argument iteratorKind (eithersyncorasync). It performs the following steps when called:

  1. If iteratorKind is not present, set iteratorKind tosync.
  2. Let oldEnv be therunning execution context's LexicalEnvironment.
  3. Let V beundefined.
  4. Let destructuring beIsDestructuringof lhs.
  5. If destructuring istrueand if lhsKind isassignment, then
    1. Assert: lhs is aLeftHandSideExpression.
    2. Let assignmentPattern be theAssignmentPatternthat iscoveredby lhs.
  6. Repeat,
    1. Let nextResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]]).
    2. If iteratorKind isasync, set nextResult to ? Await(nextResult).
    3. IfType(nextResult) is not Object, throw aTypeErrorexception.
    4. Let done be ? IteratorComplete(nextResult).
    5. If done istrue, returnNormalCompletion(V).
    6. Let nextValue be ? IteratorValue(nextResult).
    7. If lhsKind is eitherassignmentorvarBinding, then
      1. If destructuring isfalse, then
        1. Let lhsRef be the result of evaluating lhs. (It may be evaluated repeatedly.)
    8. Else,
      1. Assert: lhsKind islexicalBinding.
      2. Assert: lhs is aForDeclaration.
      3. Let iterationEnv beNewDeclarativeEnvironment(oldEnv).
      4. PerformForDeclarationBindingInstantiationfor lhs passing iterationEnv as the argument.
      5. Set therunning execution context's LexicalEnvironment to iterationEnv.
      6. If destructuring isfalse, then
        1. Assert: lhs binds a single name.
        2. Let lhsName be the sole element ofBoundNamesof lhs.
        3. Let lhsRef be ! ResolveBinding(lhsName).
    9. If destructuring isfalse, then
      1. If lhsRef is anabrupt completion, then
        1. Let status be lhsRef.
      2. Else if lhsKind islexicalBinding, then
        1. Let status beInitializeReferencedBinding(lhsRef, nextValue).
      3. Else,
        1. Let status bePutValue(lhsRef, nextValue).
    10. Else,
      1. If lhsKind isassignment, then
        1. Let status beDestructuringAssignmentEvaluationof assignmentPattern with argument nextValue.
      2. Else if lhsKind isvarBinding, then
        1. Assert: lhs is aForBinding.
        2. Let status beBindingInitializationof lhs with arguments nextValue andundefined.
      3. Else,
        1. Assert: lhsKind islexicalBinding.
        2. Assert: lhs is aForDeclaration.
        3. Let status beForDeclarationBindingInitializationof lhs with arguments nextValue and iterationEnv.
    11. If status is anabrupt completion, then
      1. Set therunning execution context's LexicalEnvironment to oldEnv.
      2. If iteratorKind isasync, return ? AsyncIteratorClose(iteratorRecord, status).
      3. If iterationKind isenumerate, then
        1. Return status.
      4. Else,
        1. Assert: iterationKind isiterate.
        2. Return ? IteratorClose(iteratorRecord, status).
    12. Let result be the result of evaluating stmt.
    13. Set therunning execution context's LexicalEnvironment to oldEnv.
    14. IfLoopContinues(result, labelSet) isfalse, then
      1. If iterationKind isenumerate, then
        1. ReturnCompletion(UpdateEmpty(result, V)).
      2. Else,
        1. Assert: iterationKind isiterate.
        2. Set status toUpdateEmpty(result, V).
        3. If iteratorKind isasync, return ? AsyncIteratorClose(iteratorRecord, status).
        4. Return ? IteratorClose(iteratorRecord, status).
    15. If result.[[Value]] is notempty, set V to result.[[Value]].

14.7.5.8 Runtime Semantics: Evaluation

ForBinding:BindingIdentifier
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Return ? ResolveBinding(bindingId).

14.7.5.9 EnumerateObjectProperties ( O )

The abstract operation EnumerateObjectProperties takes argument O. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Return an Iterator object (27.1.1.2) whose next method iterates over all the String-valued keys of enumerable properties of O. The iterator object is never directly accessible to ECMAScript code. The mechanics and order of enumerating the properties is not specified but must conform to the rules specified below.

The iterator's throw and return methods arenulland are never invoked. The iterator's next method processes object properties to determine whether the property key should be returned as an iterator value. Returned property keys do not include keys that are Symbols. Properties of the target object may be deleted during enumeration. A property that is deleted before it is processed by the iterator's next method is ignored. If new properties are added to the target object during enumeration, the newly added properties are not guaranteed to be processed in the active enumeration. Aproperty namewill be returned by the iterator's next method at most once in any enumeration.

Enumerating the properties of the target object includes enumerating properties of its prototype, and the prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it has the same name as a property that has already been processed by the iterator's next method. The values of [[Enumerable]] attributes are not considered when determining if a property of a prototype object has already been processed. The enumerable property names of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object as the argument. EnumerateObjectProperties must obtain the own property keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.

In addition, if neither O nor any object in its prototype chain is aProxy exotic object,Integer-Indexed exotic object,module namespace exotic object, or implementation providedexotic object, then the iterator must behave as would the iterator given byCreateForInIterator(O) until one of the following occurs:

  • the value of the [[Prototype]] internal slot of O or an object in its prototype chain changes,
  • a property is removed from O or an object in its prototype chain,
  • a property is added to an object in O's prototype chain, or
  • the value of the [[Enumerable]] attribute of a property of O or an object in its prototype chain changes.
Note 1

ECMAScript implementations are not required to implement the algorithm in14.7.5.10.2.1directly. They may choose any implementation whose behaviour will not deviate from that algorithm unless one of the constraints in the previous paragraph is violated.

The following is an informative definition of an ECMAScript generator function that conforms to these rules:

function* EnumerateObjectProperties(obj) {
  const visited = new Set();
  for (const key of Reflect.ownKeys(obj)) {
    if (typeof key === "symbol") continue;
    const desc = Reflect.getOwnPropertyDescriptor(obj, key);
    if (desc) {
      visited.add(key);
      if (desc.enumerable) yield key;
    }
  }
  const proto = Reflect.getPrototypeOf(obj);
  if (proto === null) return;
  for (const protoKey of EnumerateObjectProperties(proto)) {
    if (!visited.has(protoKey)) yield protoKey;
  }
}
Note 2
The list of exotic objects for which implementations are not required to matchCreateForInIteratorwas chosen because implementations historically differed in behaviour for those cases, and agreed in all others.

14.7.5.10 For-In Iterator Objects

A For-In Iterator is an object that represents a specific iteration over some specific object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the behaviour ofEnumerateObjectProperties.

14.7.5.10.1 CreateForInIterator ( object )

The abstract operation CreateForInIterator takes argument object. It is used to create a For-In Iterator object which iterates over the own and inherited enumerable string properties of object in a specific order. It performs the following steps when called:

  1. Assert:Type(object) is Object.
  2. Let iterator be ! OrdinaryObjectCreate(%ForInIteratorPrototype%, « [[Object]], [[ObjectWasVisited]], [[VisitedKeys]], [[RemainingKeys]] »).
  3. Set iterator.[[Object]] to object.
  4. Set iterator.[[ObjectWasVisited]] tofalse.
  5. Set iterator.[[VisitedKeys]] to a new emptyList.
  6. Set iterator.[[RemainingKeys]] to a new emptyList.
  7. Return iterator.

14.7.5.10.2 The %ForInIteratorPrototype% Object

The %ForInIteratorPrototype% object:

  • has properties that are inherited by all For-In Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • is never directly accessible to ECMAScript code.
  • has the following properties:

14.7.5.10.2.1 %ForInIteratorPrototype%.next ( )

  1. Let O be thethisvalue.
  2. Assert:Type(O) is Object.
  3. Assert: O has all of the internal slots of a For-In Iterator Instance (14.7.5.10.3).
  4. Let object be O.[[Object]].
  5. Let visited be O.[[VisitedKeys]].
  6. Let remaining be O.[[RemainingKeys]].
  7. Repeat,
    1. If O.[[ObjectWasVisited]] isfalse, then
      1. Let keys be ? object.[[OwnPropertyKeys]]().
      2. For each element key of keys, do
        1. IfType(key) is String, then
          1. Append key to remaining.
      3. Set O.[[ObjectWasVisited]] totrue.
    2. Repeat, while remaining is not empty,
      1. Let r be the first element of remaining.
      2. Remove the first element from remaining.
      3. If there does not exist an element v of visited such thatSameValue(r, v) istrue, then
        1. Let desc be ? object.[[GetOwnProperty]](r).
        2. If desc is notundefined, then
          1. Append r to visited.
          2. If desc.[[Enumerable]] istrue, returnCreateIterResultObject(r,false).
    3. Set object to ? object.[[GetPrototypeOf]]().
    4. Set O.[[Object]] to object.
    5. Set O.[[ObjectWasVisited]] tofalse.
    6. If object isnull, returnCreateIterResultObject(undefined,true).

14.7.5.10.3 Properties of For-In Iterator Instances

For-In Iterator instances are ordinary objects that inherit properties from the%ForInIteratorPrototype%intrinsic object. For-In Iterator instances are initially created with the internal slots listed inTable 42.

Table 42: Internal Slots of For-In Iterator Instances
Internal SlotDescription
[[Object]]The Object value whose properties are being iterated.
[[ObjectWasVisited]]trueif the iterator has invoked [[OwnPropertyKeys]] on [[Object]],falseotherwise.
[[VisitedKeys]]A list of String values which have been emitted by this iterator thus far.
[[RemainingKeys]]A list of String values remaining to be emitted for the current object, before iterating the properties of its prototype (if its prototype is notnull).

14.8 The continue Statement

Syntax

ContinueStatement[Yield, Await]:continue;continue[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];

14.8.1 Static Semantics: Early Errors

ContinueStatement:continue;continueLabelIdentifier;

14.8.2 Runtime Semantics: Evaluation

ContinueStatement:continue;
  1. ReturnCompletion{ [[Type]]:continue, [[Value]]:empty, [[Target]]:empty}.
ContinueStatement:continueLabelIdentifier;
  1. Let label be theStringValueofLabelIdentifier.
  2. ReturnCompletion{ [[Type]]:continue, [[Value]]:empty, [[Target]]: label }.

14.9 The break Statement

Syntax

BreakStatement[Yield, Await]:break;break[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];

14.9.1 Static Semantics: Early Errors

BreakStatement:break;

14.9.2 Runtime Semantics: Evaluation

BreakStatement:break;
  1. ReturnCompletion{ [[Type]]:break, [[Value]]:empty, [[Target]]:empty}.
BreakStatement:breakLabelIdentifier;
  1. Let label be theStringValueofLabelIdentifier.
  2. ReturnCompletion{ [[Type]]:break, [[Value]]:empty, [[Target]]: label }.

14.10 The return Statement

Syntax

ReturnStatement[Yield, Await]:return;return[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];Note

A return statement causes a function to cease execution and, in most cases, returns a value to the caller. IfExpressionis omitted, the return value isundefined. Otherwise, the return value is the value ofExpression. A return statement may not actually return a value to the caller depending on surrounding context. For example, in a try block, a return statement's completion record may be replaced with another completion record during evaluation of the finally block.

14.10.1 Runtime Semantics: Evaluation

ReturnStatement:return;
  1. ReturnCompletion{ [[Type]]:return, [[Value]]:undefined, [[Target]]:empty}.
ReturnStatement:returnExpression;
  1. Let exprRef be the result of evaluatingExpression.
  2. Let exprValue be ? GetValue(exprRef).
  3. If ! GetGeneratorKind() isasync, set exprValue to ? Await(exprValue).
  4. ReturnCompletion{ [[Type]]:return, [[Value]]: exprValue, [[Target]]:empty}.

14.11 The with Statement

Syntax

WithStatement[Yield, Await, Return]:with(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]Note

The with statement adds anobject Environment Recordfor a computed object to the lexical environment of therunning execution context. It then executes a statement using this augmented lexical environment. Finally, it restores the original lexical environment.

14.11.1 Static Semantics: Early Errors

WithStatement:with(Expression)StatementNote

It is only necessary to apply the second rule if the extension specified inB.3.1is implemented.

14.11.2 Runtime Semantics: Evaluation

WithStatement:with(Expression)Statement
  1. Let val be the result of evaluatingExpression.
  2. Let obj be ? ToObject(?GetValue(val)).
  3. Let oldEnv be therunning execution context's LexicalEnvironment.
  4. Let newEnv beNewObjectEnvironment(obj,true, oldEnv).
  5. Set therunning execution context's LexicalEnvironment to newEnv.
  6. Let C be the result of evaluatingStatement.
  7. Set therunning execution context's LexicalEnvironment to oldEnv.
  8. ReturnCompletion(UpdateEmpty(C,undefined)).
Note

No matter how control leaves the embeddedStatement, whether normally or by some form ofabrupt completionor exception, the LexicalEnvironment is always restored to its former state.

14.12 The switch Statement

Syntax

SwitchStatement[Yield, Await, Return]:switch(Expression[+In, ?Yield, ?Await])CaseBlock[?Yield, ?Await, ?Return]CaseBlock[Yield, Await, Return]:{CaseClauses[?Yield, ?Await, ?Return]opt}{CaseClauses[?Yield, ?Await, ?Return]optDefaultClause[?Yield, ?Await, ?Return]CaseClauses[?Yield, ?Await, ?Return]opt}CaseClauses[Yield, Await, Return]:CaseClause[?Yield, ?Await, ?Return]CaseClauses[?Yield, ?Await, ?Return]CaseClause[?Yield, ?Await, ?Return]CaseClause[Yield, Await, Return]:caseExpression[+In, ?Yield, ?Await]:StatementList[?Yield, ?Await, ?Return]optDefaultClause[Yield, Await, Return]:default:StatementList[?Yield, ?Await, ?Return]opt

14.12.1 Static Semantics: Early Errors

SwitchStatement:switch(Expression)CaseBlock

14.12.2 Runtime Semantics: CaseBlockEvaluation

With parameter input.

CaseBlock:{}
  1. ReturnNormalCompletion(undefined).
CaseBlock:{CaseClauses}
  1. Let V beundefined.
  2. Let A be theListofCaseClauseitems inCaseClauses, in source text order.
  3. Let found befalse.
  4. For eachCaseClauseC of A, do
    1. If found isfalse, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found istrue, then
      1. Let R be the result of evaluating C.
      2. If R.[[Value]] is notempty, set V to R.[[Value]].
      3. If R is anabrupt completion, returnCompletion(UpdateEmpty(R, V)).
  5. ReturnNormalCompletion(V).
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. Let V beundefined.
  2. If the firstCaseClausesis present, then
    1. Let A be theListofCaseClauseitems in the firstCaseClauses, in source text order.
  3. Else,
    1. Let A be « ».
  4. Let found befalse.
  5. For eachCaseClauseC of A, do
    1. If found isfalse, then
      1. Set found to ? CaseClauseIsSelected(C, input).
    2. If found istrue, then
      1. Let R be the result of evaluating C.
      2. If R.[[Value]] is notempty, set V to R.[[Value]].
      3. If R is anabrupt completion, returnCompletion(UpdateEmpty(R, V)).
  6. Let foundInB befalse.
  7. If the secondCaseClausesis present, then
    1. Let B be theListofCaseClauseitems in the secondCaseClauses, in source text order.
  8. Else,
    1. Let B be « ».
  9. If found isfalse, then
    1. For eachCaseClauseC of B, do
      1. If foundInB isfalse, then
        1. Set foundInB to ? CaseClauseIsSelected(C, input).
      2. If foundInB istrue, then
        1. Let R be the result of evaluatingCaseClauseC.
        2. If R.[[Value]] is notempty, set V to R.[[Value]].
        3. If R is anabrupt completion, returnCompletion(UpdateEmpty(R, V)).
  10. If foundInB istrue, returnNormalCompletion(V).
  11. Let R be the result of evaluatingDefaultClause.
  12. If R.[[Value]] is notempty, set V to R.[[Value]].
  13. If R is anabrupt completion, returnCompletion(UpdateEmpty(R, V)).
  14. NOTE: The following is another complete iteration of the secondCaseClauses.
  15. For eachCaseClauseC of B, do
    1. Let R be the result of evaluatingCaseClauseC.
    2. If R.[[Value]] is notempty, set V to R.[[Value]].
    3. If R is anabrupt completion, returnCompletion(UpdateEmpty(R, V)).
  16. ReturnNormalCompletion(V).

14.12.3 CaseClauseIsSelected ( C, input )

The abstract operation CaseClauseIsSelected takes arguments C (aParse NodeforCaseClause) and input (anECMAScript language value). It determines whether C matches input. It performs the following steps when called:

  1. Assert: C is an instance of the productionCaseClause:caseExpression:StatementListopt.
  2. Let exprRef be the result of evaluating theExpressionof C.
  3. Let clauseSelector be ? GetValue(exprRef).
  4. ReturnIsStrictlyEqual(input, clauseSelector).
Note

This operation does not execute C'sStatementList(if any). TheCaseBlockalgorithm uses its return value to determine whichStatementListto start executing.

14.12.4 Runtime Semantics: Evaluation

SwitchStatement:switch(Expression)CaseBlock
  1. Let exprRef be the result of evaluatingExpression.
  2. Let switchValue be ? GetValue(exprRef).
  3. Let oldEnv be therunning execution context's LexicalEnvironment.
  4. Let blockEnv beNewDeclarativeEnvironment(oldEnv).
  5. PerformBlockDeclarationInstantiation(CaseBlock, blockEnv).
  6. Set therunning execution context's LexicalEnvironment to blockEnv.
  7. Let R beCaseBlockEvaluationofCaseBlockwith argument switchValue.
  8. Set therunning execution context's LexicalEnvironment to oldEnv.
  9. Return R.
Note

No matter how control leaves theSwitchStatementthe LexicalEnvironment is always restored to its former state.

CaseClause:caseExpression:
  1. ReturnNormalCompletion(empty).
CaseClause:caseExpression:StatementList
  1. Return the result of evaluatingStatementList.
DefaultClause:default:
  1. ReturnNormalCompletion(empty).
DefaultClause:default:StatementList
  1. Return the result of evaluatingStatementList.

14.13 Labelled Statements

Syntax

LabelledStatement[Yield, Await, Return]:LabelIdentifier[?Yield, ?Await]:LabelledItem[?Yield, ?Await, ?Return]LabelledItem[Yield, Await, Return]:Statement[?Yield, ?Await, ?Return]FunctionDeclaration[?Yield, ?Await, ~Default]Note

AStatementmay be prefixed by a label. Labelled statements are only used in conjunction with labelled break and continue statements. ECMAScript has no goto statement. AStatementcan be part of aLabelledStatement, which itself can be part of aLabelledStatement, and so on. The labels introduced this way are collectively referred to as the “current label set” when describing the semantics of individual statements.

14.13.1 Static Semantics: Early Errors

LabelledItem:FunctionDeclaration
  • It is a Syntax Error if any source text matches this rule.
Note

An alternative definition for this rule is provided inB.3.1.

14.13.2 Static Semantics: IsLabelledFunction ( stmt )

The abstract operation IsLabelledFunction takes argument stmt. It performs the following steps when called:

  1. If stmt is not aLabelledStatement, returnfalse.
  2. Let item be theLabelledItemof stmt.
  3. If item isLabelledItem:FunctionDeclaration, returntrue.
  4. Let subStmt be theStatementof item.
  5. ReturnIsLabelledFunction(subStmt).

14.13.3 Runtime Semantics: Evaluation

LabelledStatement:LabelIdentifier:LabelledItem
  1. Let newLabelSet be a new emptyList.
  2. ReturnLabelledEvaluationof thisLabelledStatementwith argument newLabelSet.

14.13.4 Runtime Semantics: LabelledEvaluation

With parameter labelSet.

BreakableStatement:IterationStatement
  1. Let stmtResult beLoopEvaluationofIterationStatementwith argument labelSet.
  2. If stmtResult.[[Type]] isbreak, then
    1. If stmtResult.[[Target]] isempty, then
      1. If stmtResult.[[Value]] isempty, set stmtResult toNormalCompletion(undefined).
      2. Else, set stmtResult toNormalCompletion(stmtResult.[[Value]]).
  3. ReturnCompletion(stmtResult).
BreakableStatement:SwitchStatement
  1. Let stmtResult be the result of evaluatingSwitchStatement.
  2. If stmtResult.[[Type]] isbreak, then
    1. If stmtResult.[[Target]] isempty, then
      1. If stmtResult.[[Value]] isempty, set stmtResult toNormalCompletion(undefined).
      2. Else, set stmtResult toNormalCompletion(stmtResult.[[Value]]).
  3. ReturnCompletion(stmtResult).
Note 1

ABreakableStatementis one that can be exited via an unlabelledBreakStatement.

LabelledStatement:LabelIdentifier:LabelledItem
  1. Let label be theStringValueofLabelIdentifier.
  2. Let newLabelSet be thelist-concatenationof labelSet and « label ».
  3. Let stmtResult beLabelledEvaluationofLabelledItemwith argument newLabelSet.
  4. If stmtResult.[[Type]] isbreakandSameValue(stmtResult.[[Target]], label) istrue, then
    1. Set stmtResult toNormalCompletion(stmtResult.[[Value]]).
  5. ReturnCompletion(stmtResult).
LabelledItem:FunctionDeclaration
  1. Return the result of evaluatingFunctionDeclaration.
Statement:BlockStatementVariableStatementEmptyStatementExpressionStatementIfStatementContinueStatementBreakStatementReturnStatementWithStatementThrowStatementTryStatementDebuggerStatement
  1. Return the result of evaluatingStatement.
Note 2

The only two productions ofStatementwhich have special semantics for LabelledEvaluation areBreakableStatementandLabelledStatement.

14.14 The throw Statement

Syntax

ThrowStatement[Yield, Await]:throw[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];

14.14.1 Runtime Semantics: Evaluation

ThrowStatement:throwExpression;
  1. Let exprRef be the result of evaluatingExpression.
  2. Let exprValue be ? GetValue(exprRef).
  3. ReturnThrowCompletion(exprValue).

14.15 The try Statement

Syntax

TryStatement[Yield, Await, Return]:tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]tryBlock[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return]tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return]Catch[Yield, Await, Return]:catch(CatchParameter[?Yield, ?Await])Block[?Yield, ?Await, ?Return]catchBlock[?Yield, ?Await, ?Return]Finally[Yield, Await, Return]:finallyBlock[?Yield, ?Await, ?Return]CatchParameter[Yield, Await]:BindingIdentifier[?Yield, ?Await]BindingPattern[?Yield, ?Await]Note

The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime error or a throw statement. The catch clause provides the exception-handling code. When a catch clause catches an exception, itsCatchParameteris bound to that exception.

14.15.1 Static Semantics: Early Errors

Catch:catch(CatchParameter)BlockNote

An alternativestatic semanticsfor this production is given inB.3.4.

14.15.2 Runtime Semantics: CatchClauseEvaluation

With parameter thrownValue.

Catch:catch(CatchParameter)Block
  1. Let oldEnv be therunning execution context's LexicalEnvironment.
  2. Let catchEnv beNewDeclarativeEnvironment(oldEnv).
  3. For each element argName of theBoundNamesofCatchParameter, do
    1. Perform ! catchEnv.CreateMutableBinding(argName,false).
  4. Set therunning execution context's LexicalEnvironment to catchEnv.
  5. Let status beBindingInitializationofCatchParameterwith arguments thrownValue and catchEnv.
  6. If status is anabrupt completion, then
    1. Set therunning execution context's LexicalEnvironment to oldEnv.
    2. ReturnCompletion(status).
  7. Let B be the result of evaluatingBlock.
  8. Set therunning execution context's LexicalEnvironment to oldEnv.
  9. ReturnCompletion(B).
Catch:catchBlock
  1. Return the result of evaluatingBlock.
Note

No matter how control leaves theBlockthe LexicalEnvironment is always restored to its former state.

14.15.3 Runtime Semantics: Evaluation

TryStatement:tryBlockCatch
  1. Let B be the result of evaluatingBlock.
  2. If B.[[Type]] isthrow, let C beCatchClauseEvaluationofCatchwith argument B.[[Value]].
  3. Else, let C be B.
  4. ReturnCompletion(UpdateEmpty(C,undefined)).
TryStatement:tryBlockFinally
  1. Let B be the result of evaluatingBlock.
  2. Let F be the result of evaluatingFinally.
  3. If F.[[Type]] isnormal, set F to B.
  4. ReturnCompletion(UpdateEmpty(F,undefined)).
TryStatement:tryBlockCatchFinally
  1. Let B be the result of evaluatingBlock.
  2. If B.[[Type]] isthrow, let C beCatchClauseEvaluationofCatchwith argument B.[[Value]].
  3. Else, let C be B.
  4. Let F be the result of evaluatingFinally.
  5. If F.[[Type]] isnormal, set F to C.
  6. ReturnCompletion(UpdateEmpty(F,undefined)).

14.16 The debugger Statement

Syntax

DebuggerStatement:debugger;

14.16.1 Runtime Semantics: Evaluation

Note

Evaluating aDebuggerStatementmay allow an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or active this statement has no observable effect.

DebuggerStatement:debugger;
  1. If animplementation-defineddebugging facility is available and enabled, then
    1. Perform animplementation-defineddebugging action.
    2. Let result be animplementation-definedCompletionvalue.
  2. Else,
    1. Let result beNormalCompletion(empty).
  3. Return result.

15 ECMAScript Language: Functions and Classes

Note

Various ECMAScript language elements cause the creation of ECMAScript function objects (10.2). Evaluation of such functions starts with the execution of their [[Call]] internal method (10.2.1).

15.1 Parameter Lists

Syntax

UniqueFormalParameters[Yield, Await]:FormalParameters[?Yield, ?Await]FormalParameters[Yield, Await]:[empty]FunctionRestParameter[?Yield, ?Await]FormalParameterList[?Yield, ?Await]FormalParameterList[?Yield, ?Await],FormalParameterList[?Yield, ?Await],FunctionRestParameter[?Yield, ?Await]FormalParameterList[Yield, Await]:FormalParameter[?Yield, ?Await]FormalParameterList[?Yield, ?Await],FormalParameter[?Yield, ?Await]FunctionRestParameter[Yield, Await]:BindingRestElement[?Yield, ?Await]FormalParameter[Yield, Await]:BindingElement[?Yield, ?Await]

15.1.1 Static Semantics: Early Errors

UniqueFormalParameters:FormalParametersFormalParameters:FormalParameterListNote

Multiple occurrences of the sameBindingIdentifierin aFormalParameterListis only allowed for functions which have simple parameter lists and which are not defined instrict mode code.

15.1.2 Static Semantics: ContainsExpression

ObjectBindingPattern:{}{BindingRestProperty}
  1. Returnfalse.
ObjectBindingPattern:{BindingPropertyList,BindingRestProperty}
  1. ReturnContainsExpressionofBindingPropertyList.
ArrayBindingPattern:[Elisionopt]
  1. Returnfalse.
ArrayBindingPattern:[ElisionoptBindingRestElement]
  1. ReturnContainsExpressionofBindingRestElement.
ArrayBindingPattern:[BindingElementList,Elisionopt]
  1. ReturnContainsExpressionofBindingElementList.
ArrayBindingPattern:[BindingElementList,ElisionoptBindingRestElement]
  1. Let has beContainsExpressionofBindingElementList.
  2. If has istrue, returntrue.
  3. ReturnContainsExpressionofBindingRestElement.
BindingPropertyList:BindingPropertyList,BindingProperty
  1. Let has beContainsExpressionofBindingPropertyList.
  2. If has istrue, returntrue.
  3. ReturnContainsExpressionofBindingProperty.
BindingElementList:BindingElementList,BindingElisionElement
  1. Let has beContainsExpressionofBindingElementList.
  2. If has istrue, returntrue.
  3. ReturnContainsExpressionofBindingElisionElement.
BindingElisionElement:ElisionoptBindingElement
  1. ReturnContainsExpressionofBindingElement.
BindingProperty:PropertyName:BindingElement
  1. Let has beIsComputedPropertyKeyofPropertyName.
  2. If has istrue, returntrue.
  3. ReturnContainsExpressionofBindingElement.
BindingElement:BindingPatternInitializer
  1. Returntrue.
SingleNameBinding:BindingIdentifier
  1. Returnfalse.
SingleNameBinding:BindingIdentifierInitializer
  1. Returntrue.
BindingRestElement:...BindingIdentifier
  1. Returnfalse.
BindingRestElement:...BindingPattern
  1. ReturnContainsExpressionofBindingPattern.
FormalParameters:[empty]
  1. Returnfalse.
FormalParameters:FormalParameterList,FunctionRestParameter
  1. IfContainsExpressionofFormalParameterLististrue, returntrue.
  2. ReturnContainsExpressionofFunctionRestParameter.
FormalParameterList:FormalParameterList,FormalParameter
  1. IfContainsExpressionofFormalParameterLististrue, returntrue.
  2. ReturnContainsExpressionofFormalParameter.
ArrowParameters:BindingIdentifier
  1. Returnfalse.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnContainsExpressionof formals.
AsyncArrowBindingIdentifier:BindingIdentifier
  1. Returnfalse.

15.1.3 Static Semantics: IsSimpleParameterList

BindingElement:BindingPattern
  1. Returnfalse.
BindingElement:BindingPatternInitializer
  1. Returnfalse.
SingleNameBinding:BindingIdentifier
  1. Returntrue.
SingleNameBinding:BindingIdentifierInitializer
  1. Returnfalse.
FormalParameters:[empty]
  1. Returntrue.
FormalParameters:FunctionRestParameter
  1. Returnfalse.
FormalParameters:FormalParameterList,FunctionRestParameter
  1. Returnfalse.
FormalParameterList:FormalParameterList,FormalParameter
  1. IfIsSimpleParameterListofFormalParameterListisfalse, returnfalse.
  2. ReturnIsSimpleParameterListofFormalParameter.
FormalParameter:BindingElement
  1. ReturnIsSimpleParameterListofBindingElement.
ArrowParameters:BindingIdentifier
  1. Returntrue.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnIsSimpleParameterListof formals.
AsyncArrowBindingIdentifier:BindingIdentifier
  1. Returntrue.
CoverCallExpressionAndAsyncArrowHead:MemberExpressionArguments
  1. Let head be theAsyncArrowHeadthat iscoveredbyCoverCallExpressionAndAsyncArrowHead.
  2. ReturnIsSimpleParameterListof head.

15.1.4 Static Semantics: HasInitializer

BindingElement:BindingPattern
  1. Returnfalse.
BindingElement:BindingPatternInitializer
  1. Returntrue.
SingleNameBinding:BindingIdentifier
  1. Returnfalse.
SingleNameBinding:BindingIdentifierInitializer
  1. Returntrue.
FormalParameterList:FormalParameterList,FormalParameter
  1. IfHasInitializerofFormalParameterLististrue, returntrue.
  2. ReturnHasInitializerofFormalParameter.

15.1.5 Static Semantics: ExpectedArgumentCount

FormalParameters:[empty]FunctionRestParameter
  1. Return 0.
FormalParameters:FormalParameterList,FunctionRestParameter
  1. ReturnExpectedArgumentCountofFormalParameterList.
Note

The ExpectedArgumentCount of aFormalParameterListis the number ofFormalParametersto the left of either the rest parameter or the firstFormalParameterwith an Initializer. AFormalParameterwithout an initializer is allowed after the first parameter with an initializer but such parameters are considered to be optional withundefinedas their default value.

FormalParameterList:FormalParameter
  1. IfHasInitializerofFormalParameteristrue, return 0.
  2. Return 1.
FormalParameterList:FormalParameterList,FormalParameter
  1. Let count beExpectedArgumentCountofFormalParameterList.
  2. IfHasInitializerofFormalParameterLististrueorHasInitializerofFormalParameteristrue, return count.
  3. Return count + 1.
ArrowParameters:BindingIdentifier
  1. Return 1.
ArrowParameters:CoverParenthesizedExpressionAndArrowParameterList
  1. Let formals be theArrowFormalParametersthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnExpectedArgumentCountof formals.
PropertySetParameterList:FormalParameter
  1. IfHasInitializerofFormalParameteristrue, return 0.
  2. Return 1.
AsyncArrowBindingIdentifier:BindingIdentifier
  1. Return 1.

15.2 Function Definitions

Syntax

FunctionDeclaration[Yield, Await, Default]:functionBindingIdentifier[?Yield, ?Await](FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}[+Default]function(FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}FunctionExpression:functionBindingIdentifier[~Yield, ~Await]opt(FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}FunctionBody[Yield, Await]:FunctionStatementList[?Yield, ?Await]FunctionStatementList[Yield, Await]:StatementList[?Yield, ?Await, +Return]opt

15.2.1 Static Semantics: Early Errors

FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}function(FormalParameters){FunctionBody}FunctionExpression:functionBindingIdentifieropt(FormalParameters){FunctionBody}Note

TheLexicallyDeclaredNamesof aFunctionBodydoes not include identifiers bound using var or function declarations.

FunctionBody:FunctionStatementList

15.2.2 Static Semantics: FunctionBodyContainsUseStrict

FunctionBody:FunctionStatementList
  1. If theDirective PrologueofFunctionBodycontains aUse Strict Directive, returntrue; otherwise, returnfalse.

15.2.3 Runtime Semantics: EvaluateFunctionBody

With parameters functionObject and argumentsList (aList).

FunctionBody:FunctionStatementList
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Return the result of evaluatingFunctionStatementList.

15.2.4 Runtime Semantics: InstantiateOrdinaryFunctionObject

With parameters scope and privateScope.

FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}
  1. Let name beStringValueofBindingIdentifier.
  2. Let sourceText be the source text matched byFunctionDeclaration.
  3. Let F beOrdinaryFunctionCreate(%Function.prototype%, sourceText,FormalParameters,FunctionBody,non-lexical-this, scope, privateScope).
  4. PerformSetFunctionName(F, name).
  5. PerformMakeConstructor(F).
  6. Return F.
FunctionDeclaration:function(FormalParameters){FunctionBody}
  1. Let sourceText be the source text matched byFunctionDeclaration.
  2. Let F beOrdinaryFunctionCreate(%Function.prototype%, sourceText,FormalParameters,FunctionBody,non-lexical-this, scope, privateScope).
  3. PerformSetFunctionName(F,"default").
  4. PerformMakeConstructor(F).
  5. Return F.
Note

An anonymousFunctionDeclarationcan only occur as part of an export default declaration, and its function code is therefore alwaysstrict mode code.

15.2.5 Runtime Semantics: InstantiateOrdinaryFunctionExpression

With optional parameter name.

FunctionExpression:function(FormalParameters){FunctionBody}
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byFunctionExpression.
  5. Let closure beOrdinaryFunctionCreate(%Function.prototype%, sourceText,FormalParameters,FunctionBody,non-lexical-this, scope, privateScope).
  6. PerformSetFunctionName(closure, name).
  7. PerformMakeConstructor(closure).
  8. Return closure.
FunctionExpression:functionBindingIdentifier(FormalParameters){FunctionBody}
  1. Assert: name is not present.
  2. Set name toStringValueofBindingIdentifier.
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let funcEnv beNewDeclarativeEnvironment(scope).
  5. Perform funcEnv.CreateImmutableBinding(name,false).
  6. Let privateScope be therunning execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched byFunctionExpression.
  8. Let closure beOrdinaryFunctionCreate(%Function.prototype%, sourceText,FormalParameters,FunctionBody,non-lexical-this, funcEnv, privateScope).
  9. PerformSetFunctionName(closure, name).
  10. PerformMakeConstructor(closure).
  11. Perform funcEnv.InitializeBinding(name, closure).
  12. Return closure.
Note

TheBindingIdentifierin aFunctionExpressioncan be referenced from inside theFunctionExpression'sFunctionBodyto allow the function to call itself recursively. However, unlike in aFunctionDeclaration, theBindingIdentifierin aFunctionExpressioncannot be referenced from and does not affect the scope enclosing theFunctionExpression.

15.2.6 Runtime Semantics: Evaluation

FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}
  1. ReturnNormalCompletion(empty).
Note 1

An alternative semantics is provided inB.3.2.

FunctionDeclaration:function(FormalParameters){FunctionBody}
  1. ReturnNormalCompletion(empty).
FunctionExpression:functionBindingIdentifieropt(FormalParameters){FunctionBody}
  1. ReturnInstantiateOrdinaryFunctionExpressionofFunctionExpression.
Note 2

A"prototype"property is automatically created for every function defined using aFunctionDeclarationorFunctionExpression, to allow for the possibility that the function will be used as aconstructor.

FunctionStatementList:[empty]
  1. ReturnNormalCompletion(undefined).

15.3 Arrow Function Definitions

Syntax

ArrowFunction[In, Yield, Await]:ArrowParameters[?Yield, ?Await][noLineTerminatorhere]=>ConciseBody[?In]ArrowParameters[Yield, Await]:BindingIdentifier[?Yield, ?Await]CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]ConciseBody[In]:[lookahead ≠{]ExpressionBody[?In, ~Await]{FunctionBody[~Yield, ~Await]}ExpressionBody[In, Await]:AssignmentExpression[?In, ~Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
ArrowParameters[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation ofCoverParenthesizedExpressionAndArrowParameterListis refined using the following grammar:

ArrowFormalParameters[Yield, Await]:(UniqueFormalParameters[?Yield, ?Await])

15.3.1 Static Semantics: Early Errors

ArrowFunction:ArrowParameters=>ConciseBodyArrowParameters:CoverParenthesizedExpressionAndArrowParameterList

15.3.2 Static Semantics: ConciseBodyContainsUseStrict

ConciseBody:ExpressionBody
  1. Returnfalse.
ConciseBody:{FunctionBody}
  1. ReturnFunctionBodyContainsUseStrictofFunctionBody.

15.3.3 Runtime Semantics: EvaluateConciseBody

With parameters functionObject and argumentsList (aList).

ConciseBody:ExpressionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Return the result of evaluatingExpressionBody.

15.3.4 Runtime Semantics: InstantiateArrowFunctionExpression

With optional parameter name.

ArrowFunction:ArrowParameters=>ConciseBody
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byArrowFunction.
  5. Let closure beOrdinaryFunctionCreate(%Function.prototype%, sourceText,ArrowParameters,ConciseBody,lexical-this, scope, privateScope).
  6. PerformSetFunctionName(closure, name).
  7. Return closure.
Note

AnArrowFunctiondoes not define local bindings for arguments, super, this, or new.target. Any reference to arguments, super, this, or new.target within anArrowFunctionmust resolve to a binding in a lexically enclosing environment. Typically this will be the Function Environment of an immediately enclosing function. Even though anArrowFunctionmay contain references to super, thefunction objectcreated in step5is not made into a method by performingMakeMethod. AnArrowFunctionthat references super is always contained within a non-ArrowFunctionand the necessary state to implement super is accessible via the scope that is captured by thefunction objectof theArrowFunction.

15.3.5 Runtime Semantics: Evaluation

ArrowFunction:ArrowParameters=>ConciseBody
  1. ReturnInstantiateArrowFunctionExpressionofArrowFunction.
ExpressionBody:AssignmentExpression
  1. Let exprRef be the result of evaluatingAssignmentExpression.
  2. Let exprValue be ? GetValue(exprRef).
  3. ReturnCompletion{ [[Type]]:return, [[Value]]: exprValue, [[Target]]:empty}.

15.4 Method Definitions

Syntax

MethodDefinition[Yield, Await]:ClassElementName[?Yield, ?Await](UniqueFormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}GeneratorMethod[?Yield, ?Await]AsyncMethod[?Yield, ?Await]AsyncGeneratorMethod[?Yield, ?Await]getClassElementName[?Yield, ?Await](){FunctionBody[~Yield, ~Await]}setClassElementName[?Yield, ?Await](PropertySetParameterList){FunctionBody[~Yield, ~Await]}PropertySetParameterList:FormalParameter[~Yield, ~Await]

15.4.1 Static Semantics: Early Errors

MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}MethodDefinition:setClassElementName(PropertySetParameterList){FunctionBody}

15.4.2 Static Semantics: HasDirectSuper

MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}
  1. IfUniqueFormalParametersContainsSuperCallistrue, returntrue.
  2. ReturnFunctionBodyContainsSuperCall.
MethodDefinition:getClassElementName(){FunctionBody}
  1. ReturnFunctionBodyContainsSuperCall.
MethodDefinition:setClassElementName(PropertySetParameterList){FunctionBody}
  1. IfPropertySetParameterListContainsSuperCallistrue, returntrue.
  2. ReturnFunctionBodyContainsSuperCall.
GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}
  1. IfUniqueFormalParametersContainsSuperCallistrue, returntrue.
  2. ReturnGeneratorBodyContainsSuperCall.
AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}
  1. IfUniqueFormalParametersContainsSuperCallistrue, returntrue.
  2. ReturnAsyncGeneratorBodyContainsSuperCall.
AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}
  1. IfUniqueFormalParametersContainsSuperCallistrue, returntrue.
  2. ReturnAsyncFunctionBodyContainsSuperCall.

15.4.3 Static Semantics: SpecialMethod

MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}
  1. Returnfalse.
MethodDefinition:GeneratorMethodAsyncMethodAsyncGeneratorMethodgetClassElementName(){FunctionBody}setClassElementName(PropertySetParameterList){FunctionBody}
  1. Returntrue.

15.4.4 Runtime Semantics: DefineMethod

With parameter object and optional parameter functionPrototype.

MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. If functionPrototype is present, then
    1. Let prototype be functionPrototype.
  6. Else,
    1. Let prototype be%Function.prototype%.
  7. Let sourceText be the source text matched byMethodDefinition.
  8. Let closure beOrdinaryFunctionCreate(prototype, sourceText,UniqueFormalParameters,FunctionBody,non-lexical-this, scope, privateScope).
  9. PerformMakeMethod(closure, object).
  10. Return theRecord{ [[Key]]: propKey, [[Closure]]: closure }.

15.4.5 Runtime Semantics: MethodDefinitionEvaluation

With parameters object and enumerable.

MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}
  1. Let methodDef be ?DefineMethodofMethodDefinitionwith argument object.
  2. Return ? DefineMethodProperty(methodDef.[[Key]], object, methodDef.[[Closure]], enumerable).
MethodDefinition:getClassElementName(){FunctionBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. Let sourceText be the source text matched byMethodDefinition.
  6. Let formalParameterList be an instance of the productionFormalParameters:[empty].
  7. Let closure beOrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList,FunctionBody,non-lexical-this, scope, privateScope).
  8. PerformMakeMethod(closure, object).
  9. PerformSetFunctionName(closure, propKey,"get").
  10. If propKey is aPrivate Name, then
    1. ReturnPrivateElement{ [[Key]]: propKey, [[Kind]]:accessor, [[Get]]: closure, [[Set]]:undefined}.
  11. Else,
    1. Let desc be the PropertyDescriptor { [[Get]]: closure, [[Enumerable]]: enumerable, [[Configurable]]:true}.
    2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
    3. Returnempty.
MethodDefinition:setClassElementName(PropertySetParameterList){FunctionBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. Let sourceText be the source text matched byMethodDefinition.
  6. Let closure beOrdinaryFunctionCreate(%Function.prototype%, sourceText,PropertySetParameterList,FunctionBody,non-lexical-this, scope, privateScope).
  7. PerformMakeMethod(closure, object).
  8. PerformSetFunctionName(closure, propKey,"set").
  9. If propKey is aPrivate Name, then
    1. ReturnPrivateElement{ [[Key]]: propKey, [[Kind]]:accessor, [[Get]]:undefined, [[Set]]: closure }.
  10. Else,
    1. Let desc be the PropertyDescriptor { [[Set]]: closure, [[Enumerable]]: enumerable, [[Configurable]]:true}.
    2. Perform ? DefinePropertyOrThrow(object, propKey, desc).
    3. Returnempty.
GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. Let sourceText be the source text matched byGeneratorMethod.
  6. Let closure beOrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText,UniqueFormalParameters,GeneratorBody,non-lexical-this, scope, privateScope).
  7. PerformMakeMethod(closure, object).
  8. PerformSetFunctionName(closure, propKey).
  9. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  10. Perform ! DefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  11. Return ? DefineMethodProperty(propKey, object, closure, enumerable).
AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. Let sourceText be the source text matched byAsyncGeneratorMethod.
  6. Let closure be ! OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,UniqueFormalParameters,AsyncGeneratorBody,non-lexical-this, scope, privateScope).
  7. Perform ! MakeMethod(closure, object).
  8. Perform ! SetFunctionName(closure, propKey).
  9. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  10. Perform ! DefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  11. Return ? DefineMethodProperty(propKey, object, closure, enumerable).
AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}
  1. Let propKey be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(propKey).
  3. Let scope be the LexicalEnvironment of therunning execution context.
  4. Let privateScope be therunning execution context's PrivateEnvironment.
  5. Let sourceText be the source text matched byAsyncMethod.
  6. Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,UniqueFormalParameters,AsyncFunctionBody,non-lexical-this, scope, privateScope).
  7. Perform ! MakeMethod(closure, object).
  8. Return ? DefineMethodProperty(propKey, object, closure, enumerable).

15.5 Generator Function Definitions

Syntax

GeneratorMethod[Yield, Await]:*ClassElementName[?Yield, ?Await](UniqueFormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorDeclaration[Yield, Await, Default]:function*BindingIdentifier[?Yield, ?Await](FormalParameters[+Yield, ~Await]){GeneratorBody}[+Default]function*(FormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorExpression:function*BindingIdentifier[+Yield, ~Await]opt(FormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorBody:FunctionBody[+Yield, ~Await]YieldExpression[In, Await]:yieldyield[noLineTerminatorhere]AssignmentExpression[?In, +Yield, ?Await]yield[noLineTerminatorhere]*AssignmentExpression[?In, +Yield, ?Await]Note 1

The syntactic context immediately following yield requires use of theInputElementRegExpOrTemplateTaillexical goal.

Note 2

YieldExpressioncannot be used within theFormalParametersof a generator function because any expressions that are part ofFormalParametersare evaluated before the resulting generator object is in a resumable state.

Note 3

Abstract operationsrelating to generator objects are defined in27.5.3.

15.5.1 Static Semantics: Early Errors

GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}function*(FormalParameters){GeneratorBody}GeneratorExpression:function*BindingIdentifieropt(FormalParameters){GeneratorBody}

15.5.2 Runtime Semantics: EvaluateGeneratorBody

With parameters functionObject and argumentsList (aList).

GeneratorBody:FunctionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Let G be ? OrdinaryCreateFromConstructor(functionObject,"%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
  3. Set G.[[GeneratorBrand]] toempty.
  4. PerformGeneratorStart(G,FunctionBody).
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: G, [[Target]]:empty}.

15.5.3 Runtime Semantics: InstantiateGeneratorFunctionObject

With parameters scope and privateScope.

GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}
  1. Let name beStringValueofBindingIdentifier.
  2. Let sourceText be the source text matched byGeneratorDeclaration.
  3. Let F beOrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText,FormalParameters,GeneratorBody,non-lexical-this, scope, privateScope).
  4. PerformSetFunctionName(F, name).
  5. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  6. PerformDefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  7. Return F.
GeneratorDeclaration:function*(FormalParameters){GeneratorBody}
  1. Let sourceText be the source text matched byGeneratorDeclaration.
  2. Let F beOrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText,FormalParameters,GeneratorBody,non-lexical-this, scope, privateScope).
  3. PerformSetFunctionName(F,"default").
  4. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  5. PerformDefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  6. Return F.
Note

An anonymousGeneratorDeclarationcan only occur as part of an export default declaration, and its function code is therefore alwaysstrict mode code.

15.5.4 Runtime Semantics: InstantiateGeneratorFunctionExpression

With optional parameter name.

GeneratorExpression:function*(FormalParameters){GeneratorBody}
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byGeneratorExpression.
  5. Let closure beOrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText,FormalParameters,GeneratorBody,non-lexical-this, scope, privateScope).
  6. PerformSetFunctionName(closure, name).
  7. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  8. PerformDefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  9. Return closure.
GeneratorExpression:function*BindingIdentifier(FormalParameters){GeneratorBody}
  1. Assert: name is not present.
  2. Set name toStringValueofBindingIdentifier.
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let funcEnv beNewDeclarativeEnvironment(scope).
  5. Perform funcEnv.CreateImmutableBinding(name,false).
  6. Let privateScope be therunning execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched byGeneratorExpression.
  8. Let closure beOrdinaryFunctionCreate(%GeneratorFunction.prototype%, sourceText,FormalParameters,GeneratorBody,non-lexical-this, funcEnv, privateScope).
  9. PerformSetFunctionName(closure, name).
  10. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
  11. PerformDefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  12. Perform funcEnv.InitializeBinding(name, closure).
  13. Return closure.
Note

TheBindingIdentifierin aGeneratorExpressioncan be referenced from inside theGeneratorExpression'sFunctionBodyto allow the generator code to call itself recursively. However, unlike in aGeneratorDeclaration, theBindingIdentifierin aGeneratorExpressioncannot be referenced from and does not affect the scope enclosing theGeneratorExpression.

15.5.5 Runtime Semantics: Evaluation

GeneratorExpression:function*BindingIdentifieropt(FormalParameters){GeneratorBody}
  1. ReturnInstantiateGeneratorFunctionExpressionofGeneratorExpression.
YieldExpression:yield
  1. Return ? Yield(undefined).
YieldExpression:yieldAssignmentExpression
  1. Let exprRef be the result of evaluatingAssignmentExpression.
  2. Let value be ? GetValue(exprRef).
  3. Return ? Yield(value).
YieldExpression:yield*AssignmentExpression
  1. Let generatorKind be ! GetGeneratorKind().
  2. Let exprRef be the result of evaluatingAssignmentExpression.
  3. Let value be ? GetValue(exprRef).
  4. Let iteratorRecord be ? GetIterator(value, generatorKind).
  5. Let iterator be iteratorRecord.[[Iterator]].
  6. Let received beNormalCompletion(undefined).
  7. Repeat,
    1. If received.[[Type]] isnormal, then
      1. Let innerResult be ? Call(iteratorRecord.[[NextMethod]], iteratorRecord.[[Iterator]], « received.[[Value]] »).
      2. If generatorKind isasync, set innerResult to ? Await(innerResult).
      3. IfType(innerResult) is not Object, throw aTypeErrorexception.
      4. Let done be ? IteratorComplete(innerResult).
      5. If done istrue, then
        1. Return ? IteratorValue(innerResult).
      6. If generatorKind isasync, set received toAsyncGeneratorYield(?IteratorValue(innerResult)).
      7. Else, set received toGeneratorYield(innerResult).
    2. Else if received.[[Type]] isthrow, then
      1. Let throw be ? GetMethod(iterator,"throw").
      2. If throw is notundefined, then
        1. Let innerResult be ? Call(throw, iterator, « received.[[Value]] »).
        2. If generatorKind isasync, set innerResult to ? Await(innerResult).
        3. NOTE: Exceptions from the inner iterator throw method are propagated. Normal completions from an inner throw method are processed similarly to an inner next.
        4. IfType(innerResult) is not Object, throw aTypeErrorexception.
        5. Let done be ? IteratorComplete(innerResult).
        6. If done istrue, then
          1. Return ? IteratorValue(innerResult).
        7. If generatorKind isasync, set received toAsyncGeneratorYield(?IteratorValue(innerResult)).
        8. Else, set received toGeneratorYield(innerResult).
      3. Else,
        1. NOTE: If iterator does not have a throw method, this throw is going to terminate the yield* loop. But first we need to give iterator a chance to clean up.
        2. Let closeCompletion beCompletion{ [[Type]]:normal, [[Value]]:empty, [[Target]]:empty}.
        3. If generatorKind isasync, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
        4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
        5. NOTE: The next step throws aTypeErrorto indicate that there was a yield* protocol violation: iterator does not have a throw method.
        6. Throw aTypeErrorexception.
    3. Else,
      1. Assert: received.[[Type]] isreturn.
      2. Let return be ? GetMethod(iterator,"return").
      3. If return isundefined, then
        1. If generatorKind isasync, set received.[[Value]] to ? Await(received.[[Value]]).
        2. ReturnCompletion(received).
      4. Let innerReturnResult be ? Call(return, iterator, « received.[[Value]] »).
      5. If generatorKind isasync, set innerReturnResult to ? Await(innerReturnResult).
      6. IfType(innerReturnResult) is not Object, throw aTypeErrorexception.
      7. Let done be ? IteratorComplete(innerReturnResult).
      8. If done istrue, then
        1. Let value be ? IteratorValue(innerReturnResult).
        2. ReturnCompletion{ [[Type]]:return, [[Value]]: value, [[Target]]:empty}.
      9. If generatorKind isasync, set received toAsyncGeneratorYield(?IteratorValue(innerReturnResult)).
      10. Else, set received toGeneratorYield(innerReturnResult).

15.6 Async Generator Function Definitions

Syntax

AsyncGeneratorMethod[Yield, Await]:async[noLineTerminatorhere]*ClassElementName[?Yield, ?Await](UniqueFormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorDeclaration[Yield, Await, Default]:async[noLineTerminatorhere]function*BindingIdentifier[?Yield, ?Await](FormalParameters[+Yield, +Await]){AsyncGeneratorBody}[+Default]async[noLineTerminatorhere]function*(FormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorExpression:async[noLineTerminatorhere]function*BindingIdentifier[+Yield, +Await]opt(FormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorBody:FunctionBody[+Yield, +Await]Note 1

YieldExpressionandAwaitExpressioncannot be used within theFormalParametersof an async generator function because any expressions that are part ofFormalParametersare evaluated before the resulting async generator object is in a resumable state.

Note 2

Abstract operationsrelating to async generator objects are defined in27.6.3.

15.6.1 Static Semantics: Early Errors

AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}asyncfunction*(FormalParameters){AsyncGeneratorBody}AsyncGeneratorExpression:asyncfunction*BindingIdentifieropt(FormalParameters){AsyncGeneratorBody}

15.6.2 Runtime Semantics: EvaluateAsyncGeneratorBody

With parameters functionObject and argumentsList (aList).

AsyncGeneratorBody:FunctionBody
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Let generator be ? OrdinaryCreateFromConstructor(functionObject,"%AsyncGeneratorFunction.prototype.prototype%", « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] »).
  3. Set generator.[[GeneratorBrand]] toempty.
  4. Perform ! AsyncGeneratorStart(generator,FunctionBody).
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: generator, [[Target]]:empty}.

15.6.3 Runtime Semantics: InstantiateAsyncGeneratorFunctionObject

With parameters scope and privateScope.

AsyncGeneratorDeclaration:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}
  1. Let name beStringValueofBindingIdentifier.
  2. Let sourceText be the source text matched byAsyncGeneratorDeclaration.
  3. Let F be ! OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,FormalParameters,AsyncGeneratorBody,non-lexical-this, scope, privateScope).
  4. Perform ! SetFunctionName(F, name).
  5. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  6. Perform ! DefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  7. Return F.
AsyncGeneratorDeclaration:asyncfunction*(FormalParameters){AsyncGeneratorBody}
  1. Let sourceText be the source text matched byAsyncGeneratorDeclaration.
  2. Let F beOrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,FormalParameters,AsyncGeneratorBody,non-lexical-this, scope, privateScope).
  3. PerformSetFunctionName(F,"default").
  4. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  5. PerformDefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  6. Return F.
Note

An anonymousAsyncGeneratorDeclarationcan only occur as part of an export default declaration.

15.6.4 Runtime Semantics: InstantiateAsyncGeneratorFunctionExpression

With optional parameter name.

AsyncGeneratorExpression:asyncfunction*(FormalParameters){AsyncGeneratorBody}
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byAsyncGeneratorExpression.
  5. Let closure be ! OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,FormalParameters,AsyncGeneratorBody,non-lexical-this, scope, privateScope).
  6. PerformSetFunctionName(closure, name).
  7. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  8. Perform ! DefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  9. Return closure.
AsyncGeneratorExpression:asyncfunction*BindingIdentifier(FormalParameters){AsyncGeneratorBody}
  1. Assert: name is not present.
  2. Set name toStringValueofBindingIdentifier.
  3. Let scope be therunning execution context's LexicalEnvironment.
  4. Let funcEnv be ! NewDeclarativeEnvironment(scope).
  5. Perform ! funcEnv.CreateImmutableBinding(name,false).
  6. Let privateScope be therunning execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched byAsyncGeneratorExpression.
  8. Let closure be ! OrdinaryFunctionCreate(%AsyncGeneratorFunction.prototype%, sourceText,FormalParameters,AsyncGeneratorBody,non-lexical-this, funcEnv, privateScope).
  9. Perform ! SetFunctionName(closure, name).
  10. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
  11. Perform ! DefinePropertyOrThrow(closure,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  12. Perform ! funcEnv.InitializeBinding(name, closure).
  13. Return closure.
Note

TheBindingIdentifierin anAsyncGeneratorExpressioncan be referenced from inside theAsyncGeneratorExpression'sAsyncGeneratorBodyto allow the generator code to call itself recursively. However, unlike in anAsyncGeneratorDeclaration, theBindingIdentifierin anAsyncGeneratorExpressioncannot be referenced from and does not affect the scope enclosing theAsyncGeneratorExpression.

15.6.5 Runtime Semantics: Evaluation

AsyncGeneratorExpression:asyncfunction*BindingIdentifieropt(FormalParameters){AsyncGeneratorBody}
  1. ReturnInstantiateAsyncGeneratorFunctionExpressionofAsyncGeneratorExpression.

15.7 Class Definitions

Syntax

ClassDeclaration[Yield, Await, Default]:classBindingIdentifier[?Yield, ?Await]ClassTail[?Yield, ?Await][+Default]classClassTail[?Yield, ?Await]ClassExpression[Yield, Await]:classBindingIdentifier[?Yield, ?Await]optClassTail[?Yield, ?Await]ClassTail[Yield, Await]:ClassHeritage[?Yield, ?Await]opt{ClassBody[?Yield, ?Await]opt}ClassHeritage[Yield, Await]:extendsLeftHandSideExpression[?Yield, ?Await]ClassBody[Yield, Await]:ClassElementList[?Yield, ?Await]ClassElementList[Yield, Await]:ClassElement[?Yield, ?Await]ClassElementList[?Yield, ?Await]ClassElement[?Yield, ?Await]ClassElement[Yield, Await]:MethodDefinition[?Yield, ?Await]staticMethodDefinition[?Yield, ?Await]FieldDefinition[?Yield, ?Await];staticFieldDefinition[?Yield, ?Await];;FieldDefinition[Yield, Await]:ClassElementName[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optClassElementName[Yield, Await]:PropertyName[?Yield, ?Await]PrivateIdentifierNote

A class definition is alwaysstrict mode code.

15.7.1 Static Semantics: Early Errors

ClassTail:ClassHeritageopt{ClassBody}ClassBody:ClassElementListClassElement:MethodDefinitionClassElement:staticMethodDefinitionClassElement:FieldDefinition;ClassElement:staticFieldDefinition;FieldDefinition:ClassElementNameInitializeroptClassElementName:PrivateIdentifier

15.7.2 Static Semantics: ClassElementKind

ClassElement:MethodDefinition
  1. IfPropNameofMethodDefinitionis"constructor", returnConstructorMethod.
  2. ReturnNonConstructorMethod.
ClassElement:staticMethodDefinitionFieldDefinition;staticFieldDefinition;
  1. ReturnNonConstructorMethod.
ClassElement:;
  1. Returnempty.

15.7.3 Static Semantics: ConstructorMethod

ClassElementList:ClassElement
  1. IfClassElementKindofClassElementisConstructorMethod, returnClassElement.
  2. Returnempty.
ClassElementList:ClassElementListClassElement
  1. Let head beConstructorMethodofClassElementList.
  2. If head is notempty, return head.
  3. IfClassElementKindofClassElementisConstructorMethod, returnClassElement.
  4. Returnempty.
Note

Early Error rules ensure that there is only one method definition named"constructor"and that it is not anaccessor propertyor generator definition.

15.7.4 Static Semantics: IsStatic

ClassElement:MethodDefinition
  1. Returnfalse.
ClassElement:staticMethodDefinition
  1. Returntrue.
ClassElement:FieldDefinition;
  1. Returnfalse.
ClassElement:staticFieldDefinition;
  1. Returntrue.
ClassElement:;
  1. Returnfalse.

15.7.5 Static Semantics: NonConstructorElements

ClassElementList:ClassElement
  1. IfClassElementKindofClassElementisNonConstructorMethod, then
    1. Return aListwhose sole element isClassElement.
  2. Return a new emptyList.
ClassElementList:ClassElementListClassElement
  1. Let list beNonConstructorElementsofClassElementList.
  2. IfClassElementKindofClassElementisNonConstructorMethod, then
    1. AppendClassElementto the end of list.
  3. Return list.

15.7.6 Static Semantics: PrototypePropertyNameList

ClassElementList:ClassElement
  1. Let propName bePropNameofClassElement.
  2. If propName isempty, return a new emptyList.
  3. IfIsStaticofClassElementistrue, return a new emptyList.
  4. Return aListwhose sole element is propName.
ClassElementList:ClassElementListClassElement
  1. Let list bePrototypePropertyNameListofClassElementList.
  2. Let propName bePropNameofClassElement.
  3. If propName isempty, return list.
  4. IfIsStaticofClassElementistrue, return list.
  5. Return thelist-concatenationof list and « propName ».

15.7.7 Static Semantics: AllPrivateIdentifiersValid

With parameter names.

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of AllPrivateIdentifiersValid:

  1. For each child node child of thisParse Node, do
    1. If child is an instance of a nonterminal, then
      1. IfAllPrivateIdentifiersValidof child with argument names isfalse, returnfalse.
  2. Returntrue.
MemberExpression:MemberExpression.PrivateIdentifier
  1. If names contains theStringValueofPrivateIdentifier, then
    1. ReturnAllPrivateIdentifiersValidofMemberExpressionwith argument names.
  2. Returnfalse.
CallExpression:CallExpression.PrivateIdentifier
  1. If names contains theStringValueofPrivateIdentifier, then
    1. ReturnAllPrivateIdentifiersValidofCallExpressionwith argument names.
  2. Returnfalse.
OptionalChain:?.PrivateIdentifier
  1. If names contains theStringValueofPrivateIdentifier, returntrue.
  2. Returnfalse.
OptionalChain:OptionalChain.PrivateIdentifier
  1. If names contains theStringValueofPrivateIdentifier, then
    1. ReturnAllPrivateIdentifiersValidofOptionalChainwith argument names.
  2. Returnfalse.
ClassBody:ClassElementList
  1. Let newNames be thelist-concatenationof names andPrivateBoundIdentifiersofClassBody.
  2. ReturnAllPrivateIdentifiersValidofClassElementListwith argument newNames.
RelationalExpression:PrivateIdentifierinShiftExpression
  1. If names contains theStringValueofPrivateIdentifier, then
    1. ReturnAllPrivateIdentifiersValidofShiftExpressionwith argument names.
  2. Returnfalse.

15.7.8 Static Semantics: PrivateBoundIdentifiers

FieldDefinition:ClassElementNameInitializeropt
  1. ReturnPrivateBoundIdentifiersofClassElementName.
ClassElementName:PrivateIdentifier
  1. Return aListwhose sole element is theStringValueofPrivateIdentifier.
ClassElementName:PropertyNameClassElement:;
  1. Return a new emptyList.
ClassElementList:ClassElementListClassElement
  1. Let names1 bePrivateBoundIdentifiersofClassElementList.
  2. Let names2 bePrivateBoundIdentifiersofClassElement.
  3. Return thelist-concatenationof names1 and names2.
MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}getClassElementName(){FunctionBody}setClassElementName(PropertySetParameterList){FunctionBody}GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}AsyncGeneratorMethod:async*ClassElementName(UniqueFormalParameters){AsyncGeneratorBody}
  1. ReturnPrivateBoundIdentifiersofClassElementName.

15.7.9 Static Semantics: ContainsArguments

Every grammar production alternative in this specification which is not listed below implicitly has the following default definition of ContainsArguments:

  1. For each child node child of thisParse Node, do
    1. If child is an instance of a nonterminal, then
      1. IfContainsArgumentsof child istrue, returntrue.
  2. Returnfalse.
IdentifierReference:Identifier
  1. If theStringValueofIdentifieris"arguments", returntrue.
  2. Returnfalse.
FunctionDeclaration:functionBindingIdentifier(FormalParameters){FunctionBody}FunctionDeclaration:function(FormalParameters){FunctionBody}FunctionExpression:functionBindingIdentifieropt(FormalParameters){FunctionBody}
  1. Returnfalse.
MethodDefinition:ClassElementName(UniqueFormalParameters){FunctionBody}getClassElementName(){FunctionBody}setClassElementName(PropertySetParameterList){FunctionBody}
  1. ReturnContainsArgumentsofClassElementName.
GeneratorMethod:*ClassElementName(UniqueFormalParameters){GeneratorBody}
  1. ReturnContainsArgumentsofClassElementName.
GeneratorDeclaration:function*BindingIdentifier(FormalParameters){GeneratorBody}GeneratorDeclaration:function*(FormalParameters){GeneratorBody}GeneratorExpression:function*BindingIdentifieropt(FormalParameters){GeneratorBody}
  1. Returnfalse.
AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}
  1. ReturnContainsArgumentsofClassElementName.
AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}AsyncFunctionDeclaration:asyncfunction(FormalParameters){AsyncFunctionBody}AsyncFunctionExpression:asyncfunction(FormalParameters){AsyncFunctionBody}AsyncFunctionExpression:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}
  1. Returnfalse.

15.7.10 Runtime Semantics: ClassFieldDefinitionEvaluation

With parameter homeObject.

FieldDefinition:ClassElementNameInitializeropt
  1. Let name be the result of evaluatingClassElementName.
  2. ReturnIfAbrupt(name).
  3. IfInitializeroptis present, then
    1. Let formalParameterList be an instance of the productionFormalParameters:[empty].
    2. Let scope be the LexicalEnvironment of therunning execution context.
    3. Let privateScope be therunning execution context's PrivateEnvironment.
    4. Let sourceText be the empty sequence of Unicode code points.
    5. Let initializer be ! OrdinaryFunctionCreate(%Function.prototype%, sourceText, formalParameterList,Initializer,non-lexical-this, scope, privateScope).
    6. PerformMakeMethod(initializer, homeObject).
    7. Set initializer.[[ClassFieldInitializerName]] to name.
  4. Else,
    1. Let initializer beempty.
  5. Return theClassFieldDefinition Record{ [[Name]]: name, [[Initializer]]: initializer }.
Note
The function created for initializer is never directly accessible to ECMAScript code.

15.7.11 Runtime Semantics: ClassElementEvaluation

With parameter object.

ClassElement:FieldDefinition;ClassElement:staticFieldDefinition;
  1. ReturnClassFieldDefinitionEvaluationofFieldDefinitionwith argument object.
ClassElement:MethodDefinitionClassElement:staticMethodDefinition
  1. ReturnMethodDefinitionEvaluationofMethodDefinitionwith arguments object andfalse.
ClassElement:;
  1. Return.

15.7.12 Runtime Semantics: ClassDefinitionEvaluation

With parameters classBinding and className.

Note

For ease of specification, private methods and accessors are included alongside private fields in the [[PrivateElements]] slot of class instances. However, any given object has either all or none of the private methods and accessors defined by a given class. This feature has been designed so that implementations may choose to implement private methods and accessors using a strategy which does not require tracking each method or accessor individually.

For example, an implementation could directly associate instance private methods with their correspondingPrivate Nameand track, for each object, which class constructors have run with that object as their this value. Looking up an instance private method on an object then consists of checking that the classconstructorwhich defines the method has been used to initialize the object, then returning the method associated with thePrivate Name.

This differs from private fields: because field initializers can throw during class instantiation, an individual object may have some proper subset of the private fields of a given class, and so private fields must in general be tracked individually.

ClassTail:ClassHeritageopt{ClassBodyopt}
  1. Let env be the LexicalEnvironment of therunning execution context.
  2. Let classScope beNewDeclarativeEnvironment(env).
  3. If classBinding is notundefined, then
    1. Perform classScope.CreateImmutableBinding(classBinding,true).
  4. Let outerPrivateEnvironment be therunning execution context's PrivateEnvironment.
  5. Let classPrivateEnvironment beNewPrivateEnvironment(outerPrivateEnvironment).
  6. IfClassBodyoptis present, then
    1. For each String dn of thePrivateBoundIdentifiersofClassBodyopt, do
      1. If classPrivateEnvironment.[[Names]] contains aPrivate Namewhose [[Description]] is dn, then
        1. Assert: This is only possible for getter/setter pairs.
      2. Else,
        1. Let name be a newPrivate Namewhose [[Description]] value is dn.
        2. Append name to classPrivateEnvironment.[[Names]].
  7. IfClassHeritageoptis not present, then
    1. Let protoParent be%Object.prototype%.
    2. Let constructorParent be%Function.prototype%.
  8. Else,
    1. Set therunning execution context's LexicalEnvironment to classScope.
    2. NOTE: Therunning execution context's PrivateEnvironment is outerPrivateEnvironment when evaluatingClassHeritage.
    3. Let superclassRef be the result of evaluatingClassHeritage.
    4. Set therunning execution context's LexicalEnvironment to env.
    5. Let superclass be ? GetValue(superclassRef).
    6. If superclass isnull, then
      1. Let protoParent benull.
      2. Let constructorParent be%Function.prototype%.
    7. Else ifIsConstructor(superclass) isfalse, throw aTypeErrorexception.
    8. Else,
      1. Let protoParent be ? Get(superclass,"prototype").
      2. IfType(protoParent) is neither Object nor Null, throw aTypeErrorexception.
      3. Let constructorParent be superclass.
  9. Let proto be ! OrdinaryObjectCreate(protoParent).
  10. IfClassBodyoptis not present, let constructor beempty.
  11. Else, let constructor beConstructorMethodofClassBody.
  12. Set therunning execution context's LexicalEnvironment to classScope.
  13. Set therunning execution context's PrivateEnvironment to classPrivateEnvironment.
  14. If constructor isempty, then
    1. Let defaultConstructor be a newAbstract Closurewith no parameters that captures nothing and performs the following steps when called:
      1. Let args be theListof arguments that was passed to this function by [[Call]] or [[Construct]].
      2. If NewTarget isundefined, throw aTypeErrorexception.
      3. Let F be theactive function object.
      4. If F.[[ConstructorKind]] isderived, then
        1. NOTE: This branch behaves similarly to constructor(...args) { super(...args); }. The most notable distinction is that while the aforementioned ECMAScript source text observably calls the@@iteratormethod on %Array.prototype%, this function does not.
        2. Let func be ! F.[[GetPrototypeOf]]().
        3. IfIsConstructor(func) isfalse, throw aTypeErrorexception.
        4. Return ? Construct(func, args, NewTarget).
      5. Else,
        1. NOTE: This branch behaves similarly to constructor() {}.
        2. Return ? OrdinaryCreateFromConstructor(NewTarget,"%Object.prototype%").
    2. Let F be ! CreateBuiltinFunction(defaultConstructor, 0, className, « [[ConstructorKind]], [[SourceText]] »,the current Realm Record, constructorParent).
  15. Else,
    1. Let constructorInfo be !DefineMethodof constructor with arguments proto and constructorParent.
    2. Let F be constructorInfo.[[Closure]].
    3. Perform ! MakeClassConstructor(F).
    4. Perform ! SetFunctionName(F, className).
  16. Perform ! MakeConstructor(F,false, proto).
  17. IfClassHeritageoptis present, set F.[[ConstructorKind]] toderived.
  18. Perform ! CreateMethodProperty(proto,"constructor", F).
  19. IfClassBodyoptis not present, let elements be a new emptyList.
  20. Else, let elements beNonConstructorElementsofClassBody.
  21. Let instancePrivateMethods be a new emptyList.
  22. Let staticPrivateMethods be a new emptyList.
  23. Let instanceFields be a new emptyList.
  24. Let staticFields be a new emptyList.
  25. For eachClassElemente of elements, do
    1. IfIsStaticof e isfalse, then
      1. Let field beClassElementEvaluationof e with argument proto.
    2. Else,
      1. Let field beClassElementEvaluationof e with argument F.
    3. If field is anabrupt completion, then
      1. Set therunning execution context's LexicalEnvironment to env.
      2. Set therunning execution context's PrivateEnvironment to outerPrivateEnvironment.
      3. ReturnCompletion(field).
    4. Set field to field.[[Value]].
    5. If field is aPrivateElement, then
      1. Assert: field.[[Kind]] is eithermethodoraccessor.
      2. IfIsStaticof e isfalse, let container be instancePrivateMethods.
      3. Else, let container be staticPrivateMethods.
      4. If container contains aPrivateElementwhose [[Key]] is field.[[Key]], then
        1. Let existing be thatPrivateElement.
        2. Assert: field.[[Kind]] and existing.[[Kind]] are bothaccessor.
        3. If field.[[Get]] isundefined, then
          1. Let combined bePrivateElement{ [[Key]]: field.[[Key]], [[Kind]]:accessor, [[Get]]: existing.[[Get]], [[Set]]: field.[[Set]] }.
        4. Else,
          1. Let combined bePrivateElement{ [[Key]]: field.[[Key]], [[Kind]]:accessor, [[Get]]: field.[[Get]], [[Set]]: existing.[[Set]] }.
        5. Replace existing in container with combined.
      5. Else,
        1. Append field to container.
    6. Else if field is aClassFieldDefinition Record, then
      1. IfIsStaticof e isfalse, append field to instanceFields.
      2. Else, append field to staticFields.
  26. Set therunning execution context's LexicalEnvironment to env.
  27. If classBinding is notundefined, then
    1. Perform classScope.InitializeBinding(classBinding, F).
  28. Set F.[[PrivateMethods]] to instancePrivateMethods.
  29. Set F.[[Fields]] to instanceFields.
  30. For eachPrivateElementmethod of staticPrivateMethods, do
    1. Perform ! PrivateMethodOrAccessorAdd(method, F).
  31. For each element fieldRecord of staticFields, do
    1. Let result beDefineField(F, fieldRecord).
    2. If result is anabrupt completion, then
      1. Set therunning execution context's PrivateEnvironment to outerPrivateEnvironment.
      2. Return result.
  32. Set therunning execution context's PrivateEnvironment to outerPrivateEnvironment.
  33. Return F.

15.7.13 Runtime Semantics: BindingClassDeclarationEvaluation

ClassDeclaration:classBindingIdentifierClassTail
  1. Let className beStringValueofBindingIdentifier.
  2. Let value be ?ClassDefinitionEvaluationofClassTailwith arguments className and className.
  3. Set value.[[SourceText]] to the source text matched byClassDeclaration.
  4. Let env be therunning execution context's LexicalEnvironment.
  5. Perform ? InitializeBoundName(className, value, env).
  6. Return value.
ClassDeclaration:classClassTail
  1. Let value be ?ClassDefinitionEvaluationofClassTailwith argumentsundefinedand"default".
  2. Set value.[[SourceText]] to the source text matched byClassDeclaration.
  3. Return value.
Note

ClassDeclaration:classClassTailonly occurs as part of anExportDeclarationand establishing its binding is handled as part of the evaluation action for that production. See16.2.3.7.

15.7.14 Runtime Semantics: Evaluation

ClassDeclaration:classBindingIdentifierClassTail
  1. Perform ?BindingClassDeclarationEvaluationof thisClassDeclaration.
  2. ReturnNormalCompletion(empty).
Note

ClassDeclaration:classClassTailonly occurs as part of anExportDeclarationand is never directly evaluated.

ClassExpression:classClassTail
  1. Let value be ?ClassDefinitionEvaluationofClassTailwith argumentsundefinedand"".
  2. Set value.[[SourceText]] to the source text matched byClassExpression.
  3. Return value.
ClassExpression:classBindingIdentifierClassTail
  1. Let className beStringValueofBindingIdentifier.
  2. Let value be ?ClassDefinitionEvaluationofClassTailwith arguments className and className.
  3. Set value.[[SourceText]] to the source text matched byClassExpression.
  4. Return value.
ClassElementName:PrivateIdentifier
  1. Let privateIdentifier beStringValueofPrivateIdentifier.
  2. Let privateEnvRec be therunning execution context's PrivateEnvironment.
  3. Let names be privateEnvRec.[[Names]].
  4. Assert: Exactly one element of names is aPrivate Namewhose [[Description]] is privateIdentifier.
  5. Let privateName be thePrivate Namein names whose [[Description]] is privateIdentifier.
  6. Return privateName.

15.8 Async Function Definitions

Syntax

AsyncFunctionDeclaration[Yield, Await, Default]:async[noLineTerminatorhere]functionBindingIdentifier[?Yield, ?Await](FormalParameters[~Yield, +Await]){AsyncFunctionBody}[+Default]async[noLineTerminatorhere]function(FormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncFunctionExpression:async[noLineTerminatorhere]functionBindingIdentifier[~Yield, +Await]opt(FormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncMethod[Yield, Await]:async[noLineTerminatorhere]ClassElementName[?Yield, ?Await](UniqueFormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncFunctionBody:FunctionBody[~Yield, +Await]AwaitExpression[Yield]:awaitUnaryExpression[?Yield, +Await]Note 1

await is parsed as anAwaitExpressionwhen the [Await] parameter is present. The [Await] parameter is present in the following contexts:

WhenModuleis the syntacticgoal symboland the [Await] parameter is absent, await is parsed as akeywordand will be a Syntax error. WhenScriptis the syntacticgoal symbol, await may be parsed as an identifier when the [Await] parameter is absent. This includes the following contexts:

Note 2

UnlikeYieldExpression, it is a Syntax Error to omit the operand of anAwaitExpression. You must await something.

15.8.1 Static Semantics: Early Errors

AsyncMethod:asyncClassElementName(UniqueFormalParameters){AsyncFunctionBody}AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}asyncfunction(FormalParameters){AsyncFunctionBody}AsyncFunctionExpression:asyncfunctionBindingIdentifieropt(FormalParameters){AsyncFunctionBody}

15.8.2 Runtime Semantics: InstantiateAsyncFunctionObject

With parameters scope and privateScope.

AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}
  1. Let name beStringValueofBindingIdentifier.
  2. Let sourceText be the source text matched byAsyncFunctionDeclaration.
  3. Let F be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,FormalParameters,AsyncFunctionBody,non-lexical-this, scope, privateScope).
  4. Perform ! SetFunctionName(F, name).
  5. Return F.
AsyncFunctionDeclaration:asyncfunction(FormalParameters){AsyncFunctionBody}
  1. Let sourceText be the source text matched byAsyncFunctionDeclaration.
  2. Let F be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,FormalParameters,AsyncFunctionBody,non-lexical-this, scope, privateScope).
  3. Perform ! SetFunctionName(F,"default").
  4. Return F.

15.8.3 Runtime Semantics: InstantiateAsyncFunctionExpression

With optional parameter name.

AsyncFunctionExpression:asyncfunction(FormalParameters){AsyncFunctionBody}
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byAsyncFunctionExpression.
  5. Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,FormalParameters,AsyncFunctionBody,non-lexical-this, scope, privateScope).
  6. PerformSetFunctionName(closure, name).
  7. Return closure.
AsyncFunctionExpression:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}
  1. Assert: name is not present.
  2. Set name toStringValueofBindingIdentifier.
  3. Let scope be the LexicalEnvironment of therunning execution context.
  4. Let funcEnv be ! NewDeclarativeEnvironment(scope).
  5. Perform ! funcEnv.CreateImmutableBinding(name,false).
  6. Let privateScope be therunning execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched byAsyncFunctionExpression.
  8. Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText,FormalParameters,AsyncFunctionBody,non-lexical-this, funcEnv, privateScope).
  9. Perform ! SetFunctionName(closure, name).
  10. Perform ! funcEnv.InitializeBinding(name, closure).
  11. Return closure.
Note

TheBindingIdentifierin anAsyncFunctionExpressioncan be referenced from inside theAsyncFunctionExpression'sAsyncFunctionBodyto allow the function to call itself recursively. However, unlike in aFunctionDeclaration, theBindingIdentifierin aAsyncFunctionExpressioncannot be referenced from and does not affect the scope enclosing theAsyncFunctionExpression.

15.8.4 Runtime Semantics: EvaluateAsyncFunctionBody

With parameters functionObject and argumentsList (aList).

AsyncFunctionBody:FunctionBody
  1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  2. Let declResult beFunctionDeclarationInstantiation(functionObject, argumentsList).
  3. If declResult is not anabrupt completion, then
    1. Perform ! AsyncFunctionStart(promiseCapability,FunctionBody).
  4. Else,
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « declResult.[[Value]] »).
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: promiseCapability.[[Promise]], [[Target]]:empty}.

15.8.5 Runtime Semantics: Evaluation

AsyncFunctionDeclaration:asyncfunctionBindingIdentifier(FormalParameters){AsyncFunctionBody}
  1. ReturnNormalCompletion(empty).
AsyncFunctionDeclaration:asyncfunction(FormalParameters){AsyncFunctionBody}
  1. ReturnNormalCompletion(empty).
AsyncFunctionExpression:asyncfunctionBindingIdentifieropt(FormalParameters){AsyncFunctionBody}
  1. ReturnInstantiateAsyncFunctionExpressionofAsyncFunctionExpression.
AwaitExpression:awaitUnaryExpression
  1. Let exprRef be the result of evaluatingUnaryExpression.
  2. Let value be ? GetValue(exprRef).
  3. Return ? Await(value).

15.9 Async Arrow Function Definitions

Syntax

AsyncArrowFunction[In, Yield, Await]:async[noLineTerminatorhere]AsyncArrowBindingIdentifier[?Yield][noLineTerminatorhere]=>AsyncConciseBody[?In]CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await][noLineTerminatorhere]=>AsyncConciseBody[?In]AsyncConciseBody[In]:[lookahead ≠{]ExpressionBody[?In, +Await]{AsyncFunctionBody}AsyncArrowBindingIdentifier[Yield]:BindingIdentifier[?Yield, +Await]CoverCallExpressionAndAsyncArrowHead[Yield, Await]:MemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]

Supplemental Syntax

When processing an instance of the production
AsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody
the interpretation ofCoverCallExpressionAndAsyncArrowHeadis refined using the following grammar:

AsyncArrowHead:async[noLineTerminatorhere]ArrowFormalParameters[~Yield, +Await]

15.9.1 Static Semantics: Early Errors

AsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBodyAsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody

15.9.2 Static Semantics: AsyncConciseBodyContainsUseStrict

AsyncConciseBody:ExpressionBody
  1. Returnfalse.
AsyncConciseBody:{AsyncFunctionBody}
  1. ReturnFunctionBodyContainsUseStrictofAsyncFunctionBody.

15.9.3 Runtime Semantics: EvaluateAsyncConciseBody

With parameters functionObject and argumentsList (aList).

AsyncConciseBody:ExpressionBody
  1. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  2. Let declResult beFunctionDeclarationInstantiation(functionObject, argumentsList).
  3. If declResult is not anabrupt completion, then
    1. Perform ! AsyncFunctionStart(promiseCapability,ExpressionBody).
  4. Else,
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « declResult.[[Value]] »).
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: promiseCapability.[[Promise]], [[Target]]:empty}.

15.9.4 Runtime Semantics: InstantiateAsyncArrowFunctionExpression

With optional parameter name.

AsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBody
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byAsyncArrowFunction.
  5. Let parameters beAsyncArrowBindingIdentifier.
  6. Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters,AsyncConciseBody,lexical-this, scope, privateScope).
  7. PerformSetFunctionName(closure, name).
  8. Return closure.
AsyncArrowFunction:CoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody
  1. If name is not present, set name to"".
  2. Let scope be the LexicalEnvironment of therunning execution context.
  3. Let privateScope be therunning execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched byAsyncArrowFunction.
  5. Let head be theAsyncArrowHeadthat iscoveredbyCoverCallExpressionAndAsyncArrowHead.
  6. Let parameters be theArrowFormalParametersof head.
  7. Let closure be ! OrdinaryFunctionCreate(%AsyncFunction.prototype%, sourceText, parameters,AsyncConciseBody,lexical-this, scope, privateScope).
  8. PerformSetFunctionName(closure, name).
  9. Return closure.

15.9.5 Runtime Semantics: Evaluation

AsyncArrowFunction:asyncAsyncArrowBindingIdentifier=>AsyncConciseBodyCoverCallExpressionAndAsyncArrowHead=>AsyncConciseBody
  1. ReturnInstantiateAsyncArrowFunctionExpressionofAsyncArrowFunction.

15.10 Tail Position Calls

15.10.1 Static Semantics: IsInTailPosition ( call )

The abstract operation IsInTailPosition takes argument call. It performs the following steps when called:

  1. Assert: call is aParse Node.
  2. If the source code matching call isnon-strict code, returnfalse.
  3. If call is not contained within aFunctionBody,ConciseBody, orAsyncConciseBody, returnfalse.
  4. Let body be theFunctionBody,ConciseBody, orAsyncConciseBodythat most closely contains call.
  5. If body is theFunctionBodyof aGeneratorBody, returnfalse.
  6. If body is theFunctionBodyof anAsyncFunctionBody, returnfalse.
  7. If body is theFunctionBodyof anAsyncGeneratorBody, returnfalse.
  8. If body is anAsyncConciseBody, returnfalse.
  9. Return the result ofHasCallInTailPositionof body with argument call.
Note

Tail Position calls are only defined instrict mode codebecause of a common non-standard language extension (see10.2.4) that enables observation of the chain of caller contexts.

15.10.2 Static Semantics: HasCallInTailPosition

With parameter call.

Note

call is aParse Nodethat represents a specific range of source text. When the following algorithms compare call to anotherParse Node, it is a test of whether they represent the same source text.

15.10.2.1 Statement Rules

StatementList:StatementListStatementListItem
  1. Let has beHasCallInTailPositionofStatementListwith argument call.
  2. If has istrue, returntrue.
  3. ReturnHasCallInTailPositionofStatementListItemwith argument call.
FunctionStatementList:[empty]StatementListItem:DeclarationStatement:VariableStatementEmptyStatementExpressionStatementContinueStatementBreakStatementThrowStatementDebuggerStatementBlock:{}ReturnStatement:return;LabelledItem:FunctionDeclarationForInOfStatement:for(LeftHandSideExpressionofAssignmentExpression)Statementfor(varForBindingofAssignmentExpression)Statementfor(ForDeclarationofAssignmentExpression)StatementCaseBlock:{}
  1. Returnfalse.
IfStatement:if(Expression)StatementelseStatement
  1. Let has beHasCallInTailPositionof the firstStatementwith argument call.
  2. If has istrue, returntrue.
  3. ReturnHasCallInTailPositionof the secondStatementwith argument call.
IfStatement:if(Expression)StatementDoWhileStatement:doStatementwhile(Expression);WhileStatement:while(Expression)StatementForStatement:for(Expressionopt;Expressionopt;Expressionopt)Statementfor(varVariableDeclarationList;Expressionopt;Expressionopt)Statementfor(LexicalDeclarationExpressionopt;Expressionopt)StatementForInOfStatement:for(LeftHandSideExpressioninExpression)Statementfor(varForBindinginExpression)Statementfor(ForDeclarationinExpression)Statementforawait(LeftHandSideExpressionofAssignmentExpression)Statementforawait(varForBindingofAssignmentExpression)Statementforawait(ForDeclarationofAssignmentExpression)StatementWithStatement:with(Expression)Statement
  1. ReturnHasCallInTailPositionofStatementwith argument call.
LabelledStatement:LabelIdentifier:LabelledItem
  1. ReturnHasCallInTailPositionofLabelledItemwith argument call.
ReturnStatement:returnExpression;
  1. ReturnHasCallInTailPositionofExpressionwith argument call.
SwitchStatement:switch(Expression)CaseBlock
  1. ReturnHasCallInTailPositionofCaseBlockwith argument call.
CaseBlock:{CaseClausesoptDefaultClauseCaseClausesopt}
  1. Let has befalse.
  2. If the firstCaseClausesis present, let has beHasCallInTailPositionof the firstCaseClauseswith argument call.
  3. If has istrue, returntrue.
  4. Let has beHasCallInTailPositionofDefaultClausewith argument call.
  5. If has istrue, returntrue.
  6. If the secondCaseClausesis present, let has beHasCallInTailPositionof the secondCaseClauseswith argument call.
  7. Return has.
CaseClauses:CaseClausesCaseClause
  1. Let has beHasCallInTailPositionofCaseClauseswith argument call.
  2. If has istrue, returntrue.
  3. ReturnHasCallInTailPositionofCaseClausewith argument call.
CaseClause:caseExpression:StatementListoptDefaultClause:default:StatementListopt
  1. IfStatementListis present, returnHasCallInTailPositionofStatementListwith argument call.
  2. Returnfalse.
TryStatement:tryBlockCatch
  1. ReturnHasCallInTailPositionofCatchwith argument call.
TryStatement:tryBlockFinallyTryStatement:tryBlockCatchFinally
  1. ReturnHasCallInTailPositionofFinallywith argument call.
Catch:catch(CatchParameter)Block
  1. ReturnHasCallInTailPositionofBlockwith argument call.

15.10.2.2 Expression Rules

Note

A potential tail position call that is immediately followed by returnGetValueof the call result is also a possible tail position call. A function call cannot return aReference Record, so such aGetValueoperation will always return the same value as the actual function call result.

AssignmentExpression:YieldExpressionArrowFunctionAsyncArrowFunctionLeftHandSideExpression=AssignmentExpressionLeftHandSideExpressionAssignmentOperatorAssignmentExpressionLeftHandSideExpression&&=AssignmentExpressionLeftHandSideExpression||=AssignmentExpressionLeftHandSideExpression??=AssignmentExpressionBitwiseANDExpression:BitwiseANDExpression&EqualityExpressionBitwiseXORExpression:BitwiseXORExpression^BitwiseANDExpressionBitwiseORExpression:BitwiseORExpression|BitwiseXORExpressionEqualityExpression:EqualityExpression==RelationalExpressionEqualityExpression!=RelationalExpressionEqualityExpression===RelationalExpressionEqualityExpression!==RelationalExpressionRelationalExpression:RelationalExpression<ShiftExpressionRelationalExpression>ShiftExpressionRelationalExpression<=ShiftExpressionRelationalExpression>=ShiftExpressionRelationalExpressioninstanceofShiftExpressionRelationalExpressioninShiftExpressionPrivateIdentifierinShiftExpressionShiftExpression:ShiftExpression<<AdditiveExpressionShiftExpression>>AdditiveExpressionShiftExpression>>>AdditiveExpressionAdditiveExpression:AdditiveExpression+MultiplicativeExpressionAdditiveExpression-MultiplicativeExpressionMultiplicativeExpression:MultiplicativeExpressionMultiplicativeOperatorExponentiationExpressionExponentiationExpression:UpdateExpression**ExponentiationExpressionUpdateExpression:LeftHandSideExpression++LeftHandSideExpression--++UnaryExpression--UnaryExpressionUnaryExpression:deleteUnaryExpressionvoidUnaryExpressiontypeofUnaryExpression+UnaryExpression-UnaryExpression~UnaryExpression!UnaryExpressionAwaitExpressionCallExpression:SuperCallCallExpression[Expression]CallExpression.IdentifierNameCallExpression.PrivateIdentifierNewExpression:newNewExpressionMemberExpression:MemberExpression[Expression]MemberExpression.IdentifierNameSuperPropertyMetaPropertynewMemberExpressionArgumentsMemberExpression.PrivateIdentifierPrimaryExpression:thisIdentifierReferenceLiteralArrayLiteralObjectLiteralFunctionExpressionClassExpressionGeneratorExpressionAsyncFunctionExpressionAsyncGeneratorExpressionRegularExpressionLiteralTemplateLiteral
  1. Returnfalse.
Expression:AssignmentExpressionExpression,AssignmentExpression
  1. ReturnHasCallInTailPositionofAssignmentExpressionwith argument call.
ConditionalExpression:ShortCircuitExpression?AssignmentExpression:AssignmentExpression
  1. Let has beHasCallInTailPositionof the firstAssignmentExpressionwith argument call.
  2. If has istrue, returntrue.
  3. ReturnHasCallInTailPositionof the secondAssignmentExpressionwith argument call.
LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression
  1. ReturnHasCallInTailPositionofBitwiseORExpressionwith argument call.
LogicalORExpression:LogicalORExpression||LogicalANDExpression
  1. ReturnHasCallInTailPositionofLogicalANDExpressionwith argument call.
CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. ReturnHasCallInTailPositionofBitwiseORExpressionwith argument call.
CallExpression:CoverCallExpressionAndAsyncArrowHeadCallExpressionArgumentsCallExpressionTemplateLiteral
  1. If thisCallExpressionis call, returntrue.
  2. Returnfalse.
OptionalExpression:MemberExpressionOptionalChainCallExpressionOptionalChainOptionalExpressionOptionalChain
  1. ReturnHasCallInTailPositionofOptionalChainwith argument call.
OptionalChain:?.[Expression]?.IdentifierName?.PrivateIdentifierOptionalChain[Expression]OptionalChain.IdentifierNameOptionalChain.PrivateIdentifier
  1. Returnfalse.
OptionalChain:?.ArgumentsOptionalChainArguments
  1. If thisOptionalChainis call, returntrue.
  2. Returnfalse.
MemberExpression:MemberExpressionTemplateLiteral
  1. If thisMemberExpressionis call, returntrue.
  2. Returnfalse.
PrimaryExpression:CoverParenthesizedExpressionAndArrowParameterList
  1. Let expr be theParenthesizedExpressionthat iscoveredbyCoverParenthesizedExpressionAndArrowParameterList.
  2. ReturnHasCallInTailPositionof expr with argument call.
ParenthesizedExpression:(Expression)
  1. ReturnHasCallInTailPositionofExpressionwith argument call.

15.10.3 PrepareForTailCall ( )

The abstract operation PrepareForTailCall takes no arguments. It performs the following steps when called:

  1. Let leafContext be therunning execution context.
  2. Suspend leafContext.
  3. Pop leafContext from theexecution context stack. Theexecution contextnow on the top of the stack becomes therunning execution context.
  4. Assert: leafContext has no further use. It will never be activated as therunning execution context.

A tail position call must either release any transient internal resources associated with the currently executing functionexecution contextbefore invoking the target function or reuse those resources in support of the target function.

Note

For example, a tail position call should only grow an implementation's activation record stack by the amount that the size of the target function's activation record exceeds the size of the calling function's activation record. If the target function's activation record is smaller, then the total size of the stack should decrease.

16 ECMAScript Language: Scripts and Modules

16.1 Scripts

Syntax

Script:ScriptBodyoptScriptBody:StatementList[~Yield, ~Await, ~Return]

16.1.1 Static Semantics: Early Errors

Script:ScriptBodyScriptBody:StatementList

16.1.2 Static Semantics: IsStrict

Script:ScriptBodyopt
  1. IfScriptBodyis present and theDirective PrologueofScriptBodycontains aUse Strict Directive, returntrue; otherwise, returnfalse.

16.1.3 Runtime Semantics: Evaluation

Script:[empty]
  1. ReturnNormalCompletion(undefined).

16.1.4 Script Records

A Script Record encapsulates information about a script being evaluated. Each script record contains the fields listed inTable 43.

Table 43:Script RecordFields
Field NameValue TypeMeaning
[[Realm]]Realm Record|undefinedTherealmwithin which this script was created.undefinedif not yet assigned.
[[ECMAScriptCode]]aParse NodeThe result of parsing the source text of this script usingScriptas thegoal symbol.
[[HostDefined]]Any, default value isempty.Field reserved for use byhostenvironments that need to associate additional information with a script.

16.1.5 ParseScript ( sourceText, realm, hostDefined )

The abstract operation ParseScript takes arguments sourceText, realm, and hostDefined. It creates aScript Recordbased upon the result of parsing sourceText as aScript. It performs the following steps when called:

  1. Assert: sourceText is an ECMAScript source text (see clause11).
  2. Let body beParseText(sourceText,Script).
  3. If body is aListof errors, return body.
  4. ReturnScript Record{ [[Realm]]: realm, [[ECMAScriptCode]]: body, [[HostDefined]]: hostDefined }.
Note

An implementation may parse script source text and analyse it for Early Error conditions prior to evaluation of ParseScript for that script source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseScript upon that source text.

16.1.6 ScriptEvaluation ( scriptRecord )

The abstract operation ScriptEvaluation takes argument scriptRecord. It performs the following steps when called:

  1. Let globalEnv be scriptRecord.[[Realm]].[[GlobalEnv]].
  2. Let scriptContext be a new ECMAScript codeexecution context.
  3. Set the Function of scriptContext tonull.
  4. Set theRealmof scriptContext to scriptRecord.[[Realm]].
  5. Set the ScriptOrModule of scriptContext to scriptRecord.
  6. Set the VariableEnvironment of scriptContext to globalEnv.
  7. Set the LexicalEnvironment of scriptContext to globalEnv.
  8. Set the PrivateEnvironment of scriptContext tonull.
  9. Suspend the currentlyrunning execution context.
  10. Push scriptContext onto theexecution context stack; scriptContext is now therunning execution context.
  11. Let scriptBody be scriptRecord.[[ECMAScriptCode]].
  12. Let result beGlobalDeclarationInstantiation(scriptBody, globalEnv).
  13. If result.[[Type]] isnormal, then
    1. Set result to the result of evaluating scriptBody.
  14. If result.[[Type]] isnormaland result.[[Value]] isempty, then
    1. Set result toNormalCompletion(undefined).
  15. Suspend scriptContext and remove it from theexecution context stack.
  16. Assert: Theexecution context stackis not empty.
  17. Resume the context that is now on the top of theexecution context stackas therunning execution context.
  18. ReturnCompletion(result).

16.1.7 GlobalDeclarationInstantiation ( script, env )

The abstract operation GlobalDeclarationInstantiation takes arguments script (aParse NodeforScriptBody) and env (anEnvironment Record). script is theScriptBodyfor which theexecution contextis being established. env is the global environment in which bindings are to be created.

Note 1

When anexecution contextis established for evaluating scripts, declarations are instantiated in the current global environment. Each global binding declared in the code is instantiated.

It performs the following steps when called:

  1. Assert: env is aglobal Environment Record.
  2. Let lexNames be theLexicallyDeclaredNamesof script.
  3. Let varNames be theVarDeclaredNamesof script.
  4. For each element name of lexNames, do
    1. If env.HasVarDeclaration(name) istrue, throw aSyntaxErrorexception.
    2. If env.HasLexicalDeclaration(name) istrue, throw aSyntaxErrorexception.
    3. Let hasRestrictedGlobal be ? env.HasRestrictedGlobalProperty(name).
    4. If hasRestrictedGlobal istrue, throw aSyntaxErrorexception.
  5. For each element name of varNames, do
    1. If env.HasLexicalDeclaration(name) istrue, throw aSyntaxErrorexception.
  6. Let varDeclarations be theVarScopedDeclarationsof script.
  7. Let functionsToInitialize be a new emptyList.
  8. Let declaredFunctionNames be a new emptyList.
  9. For each element d of varDeclarations, in reverseListorder, do
    1. If d is neither aVariableDeclarationnor aForBindingnor aBindingIdentifier, then
      1. Assert: d is either aFunctionDeclaration, aGeneratorDeclaration, anAsyncFunctionDeclaration, or anAsyncGeneratorDeclaration.
      2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
      3. Let fn be the sole element of theBoundNamesof d.
      4. If fn is not an element of declaredFunctionNames, then
        1. Let fnDefinable be ? env.CanDeclareGlobalFunction(fn).
        2. If fnDefinable isfalse, throw aTypeErrorexception.
        3. Append fn to declaredFunctionNames.
        4. Insert d as the first element of functionsToInitialize.
  10. Let declaredVarNames be a new emptyList.
  11. For each element d of varDeclarations, do
    1. If d is aVariableDeclaration, aForBinding, or aBindingIdentifier, then
      1. For each String vn of theBoundNamesof d, do
        1. If vn is not an element of declaredFunctionNames, then
          1. Let vnDefinable be ? env.CanDeclareGlobalVar(vn).
          2. If vnDefinable isfalse, throw aTypeErrorexception.
          3. If vn is not an element of declaredVarNames, then
            1. Append vn to declaredVarNames.
  12. NOTE: No abnormal terminations occur after this algorithm step if theglobal objectis anordinary object. However, if theglobal objectis aProxy exotic objectit may exhibit behaviours that cause abnormal terminations in some of the following steps.
  13. NOTE: AnnexB.3.2.2adds additional steps at this point.
  14. Let lexDeclarations be theLexicallyScopedDeclarationsof script.
  15. Let privateEnv benull.
  16. For each element d of lexDeclarations, do
    1. NOTE: Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of theBoundNamesof d, do
      1. IfIsConstantDeclarationof d istrue, then
        1. Perform ? env.CreateImmutableBinding(dn,true).
      2. Else,
        1. Perform ? env.CreateMutableBinding(dn,false).
  17. For eachParse Nodef of functionsToInitialize, do
    1. Let fn be the sole element of theBoundNamesof f.
    2. Let fo beInstantiateFunctionObjectof f with arguments env and privateEnv.
    3. Perform ? env.CreateGlobalFunctionBinding(fn, fo,false).
  18. For each String vn of declaredVarNames, do
    1. Perform ? env.CreateGlobalVarBinding(vn,false).
  19. ReturnNormalCompletion(empty).
Note 2

Early errors specified in16.1.1prevent name conflicts between function/var declarations and let/const/class declarations as well as redeclaration of let/const/class bindings for declaration contained within a singleScript. However, such conflicts and redeclarations that span more than oneScriptare detected as runtime errors during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for the script. However, if theglobal objectis defined using Proxy exotic objects then the runtime tests for conflicting declarations may be unreliable resulting in anabrupt completionand some global declarations not being instantiated. If this occurs, the code for theScriptis not evaluated.

Unlike explicit var or function declarations, properties that are directly created on theglobal objectresult in global bindings that may be shadowed by let/const/class declarations.

16.2 Modules

Syntax

Module:ModuleBodyoptModuleBody:ModuleItemListModuleItemList:ModuleItemModuleItemListModuleItemModuleItem:ImportDeclarationExportDeclarationStatementListItem[~Yield, ~Await, ~Return]

16.2.1 Module Semantics

16.2.1.1 Static Semantics: Early Errors

ModuleBody:ModuleItemListNote

The duplicateExportedNamesrule implies that multiple export defaultExportDeclarationitems within aModuleBodyis a Syntax Error. Additional error conditions relating to conflicting or duplicate declarations are checked during module linking prior to evaluation of aModule. If any such errors are detected theModuleis not evaluated.

16.2.1.2 Static Semantics: ImportedLocalNames ( importEntries )

The abstract operation ImportedLocalNames takes argument importEntries (aListof ImportEntry Records (seeTable 49)). It creates aListof all of the local name bindings defined by importEntries. It performs the following steps when called:

  1. Let localNames be a new emptyList.
  2. For eachImportEntry Recordi of importEntries, do
    1. Append i.[[LocalName]] to localNames.
  3. Return localNames.

16.2.1.3 Static Semantics: ModuleRequests

Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItem
  1. ReturnModuleRequestsofModuleItem.
ModuleItemList:ModuleItemListModuleItem
  1. Let moduleNames beModuleRequestsofModuleItemList.
  2. Let additionalNames beModuleRequestsofModuleItem.
  3. Append to moduleNames each element of additionalNames that is not already an element of moduleNames.
  4. Return moduleNames.
ModuleItem:StatementListItem
  1. Return a new emptyList.
ImportDeclaration:importImportClauseFromClause;
  1. ReturnModuleRequestsofFromClause.
ModuleSpecifier:StringLiteral
  1. Return aListwhose sole element is theSVofStringLiteral.
ExportDeclaration:exportExportFromClauseFromClause;
  1. Return theModuleRequestsofFromClause.
ExportDeclaration:exportNamedExports;exportVariableStatementexportDeclarationexportdefaultHoistableDeclarationexportdefaultClassDeclarationexportdefaultAssignmentExpression;
  1. Return a new emptyList.

16.2.1.4 Abstract Module Records

A Module Record encapsulates structural information about the imports and exports of a single module. This information is used to link the imports and exports of sets of connected modules. A Module Record includes four fields that are only used when evaluating a module.

For specification purposes Module Record values are values of theRecordspecification type and can be thought of as existing in a simple object-oriented hierarchy where Module Record is an abstract class with both abstract and concrete subclasses. This specification defines the abstract subclass namedCyclic Module Recordand its concrete subclass namedSource Text Module Record. Other specifications and implementations may define additional Module Record subclasses corresponding to alternative module definition facilities that they defined.

Module Record defines the fields listed inTable 44. All Module Definition subclasses include at least those fields. Module Record also defines the abstract method list inTable 45. All Module definition subclasses must provide concrete implementations of these abstract methods.

Table 44:Module RecordFields
Field NameValue TypeMeaning
[[Realm]]Realm Record|undefinedTheRealmwithin which this module was created.undefinedif not yet assigned.
[[Environment]]module Environment Record|undefinedTheEnvironment Recordcontaining the top level bindings for this module. This field is set when the module is linked.
[[Namespace]]Object |undefinedThe Module Namespace Object (28.3) if one has been created for this module. Otherwiseundefined.
[[HostDefined]]Any, default value isundefined.Field reserved for use byhostenvironments that need to associate additional information with a module.
Table 45: Abstract Methods of Module Records
MethodPurpose
GetExportedNames([exportStarSet])Return a list of all names that are either directly or indirectly exported from this module.
ResolveExport(exportName [, resolveSet])

Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding Record, of the form { [[Module]]:Module Record, [[BindingName]]: String }. If the export is a Module Namespace Object without a direct binding in any module, [[BindingName]] will be set to"*namespace*". Returnnullif the name cannot be resolved, or"ambiguous"if multiple bindings were found.

Each time this operation is called with a specific exportName, resolveSet pair as arguments it must return the same result if it completes normally.

Link()

Prepare the module for evaluation by transitively resolving all module dependencies and creating amodule Environment Record.

Evaluate()

If this module has already been evaluated successfully, returnundefined; if it has already been evaluated unsuccessfully, throw the exception that was produced. Otherwise, transitively evaluate all module dependencies of this module and then evaluate this module.

Link must have completed successfully prior to invoking this method.

16.2.1.5 Cyclic Module Records

A Cyclic Module Record is used to represent information about a module that can participate in dependency cycles with other modules that are subclasses of theCyclic Module Recordtype. Module Records that are not subclasses of theCyclic Module Recordtype must not participate in dependency cycles with Source Text Module Records.

In addition to the fields defined inTable 44Cyclic Module Records have the additional fields listed inTable 46

Table 46: Additional Fields of Cyclic Module Records
Field NameValue TypeMeaning
[[Status]]unlinked|linking|linked|evaluating|evaluatedInitiallyunlinked. Transitions tolinking,linked,evaluating,evaluated(in that order) as the module progresses throughout its lifecycle.
[[EvaluationError]]Anabrupt completion|undefinedA completion of typethrowrepresenting the exception that occurred during evaluation.undefinedif no exception occurred or if [[Status]] is notevaluated.
[[DFSIndex]]Integer|undefinedAuxiliary field used during Link and Evaluate only. If [[Status]] islinkingorevaluating, this non-negative number records the point at which the module was first visited during the ongoing depth-first traversal of the dependency graph.
[[DFSAncestorIndex]]Integer|undefinedAuxiliary field used during Link and Evaluate only. If [[Status]] islinkingorevaluating, this is either the module's own [[DFSIndex]] or that of an "earlier" module in the same strongly connected component.
[[RequestedModules]]Listof StringAListof all theModuleSpecifierstrings used by the module represented by this record to request the importation of a module. TheListis source code occurrence ordered.

In addition to the methods defined inTable 45Cyclic Module Records have the additional methods listed inTable 47

Table 47: Additional Abstract Methods of Cyclic Module Records
MethodPurpose
InitializeEnvironment()Initialize theEnvironment Recordof the module, including resolving all imported bindings, and create the module'sexecution context.
ExecuteModule()Evaluate the module's code within itsexecution context.

16.2.1.5.1 Link ( )

The Link concrete method of aCyclic Module Recordmodule takes no arguments. On success, Link transitions this module's [[Status]] fromunlinkedtolinked. On failure, an exception is thrown and this module's [[Status]] remainsunlinked. (Most of the work is done by the auxiliary functionInnerModuleLinking.) It performs the following steps when called:

  1. Assert: module.[[Status]] is notlinkingorevaluating.
  2. Let stack be a new emptyList.
  3. Let result beInnerModuleLinking(module, stack, 0).
  4. If result is anabrupt completion, then
    1. For eachCyclic Module Recordm of stack, do
      1. Assert: m.[[Status]] islinking.
      2. Set m.[[Status]] tounlinked.
      3. Set m.[[Environment]] toundefined.
      4. Set m.[[DFSIndex]] toundefined.
      5. Set m.[[DFSAncestorIndex]] toundefined.
    2. Assert: module.[[Status]] isunlinked.
    3. Return result.
  5. Assert: module.[[Status]] islinkedorevaluated.
  6. Assert: stack is empty.
  7. Returnundefined.

16.2.1.5.1.1 InnerModuleLinking ( module, stack, index )

The abstract operation InnerModuleLinking takes arguments module (aCyclic Module Record), stack, and index (a non-negativeinteger). It is used by Link to perform the actual linking process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs), such that all modules in an SCC transition tolinkedtogether. It performs the following steps when called:

  1. If module is not aCyclic Module Record, then
    1. Perform ? module.Link().
    2. Return index.
  2. If module.[[Status]] islinking,linked, orevaluated, then
    1. Return index.
  3. Assert: module.[[Status]] isunlinked.
  4. Set module.[[Status]] tolinking.
  5. Set module.[[DFSIndex]] to index.
  6. Set module.[[DFSAncestorIndex]] to index.
  7. Set index to index + 1.
  8. Append module to stack.
  9. For each String required of module.[[RequestedModules]], do
    1. Let requiredModule be ? HostResolveImportedModule(module, required).
    2. Set index to ? InnerModuleLinking(requiredModule, stack, index).
    3. If requiredModule is aCyclic Module Record, then
      1. Assert: requiredModule.[[Status]] is eitherlinking,linked, orevaluated.
      2. Assert: requiredModule.[[Status]] islinkingif and only if requiredModule is in stack.
      3. If requiredModule.[[Status]] islinking, then
        1. Set module.[[DFSAncestorIndex]] tomin(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
  10. Perform ? module.InitializeEnvironment().
  11. Assert: module occurs exactly once in stack.
  12. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
  13. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
    1. Let done befalse.
    2. Repeat, while done isfalse,
      1. Let requiredModule be the last element in stack.
      2. Remove the last element of stack.
      3. Assert: requiredModule is aCyclic Module Record.
      4. Set requiredModule.[[Status]] tolinked.
      5. If requiredModule and module are the sameModule Record, set done totrue.
  14. Return index.

16.2.1.5.2 Evaluate ( )

The Evaluate concrete method of aCyclic Module Recordmodule takes no arguments. Evaluate transitions this module's [[Status]] fromlinkedtoevaluated. If execution results in an exception, that exception is recorded in the [[EvaluationError]] field and rethrown by future invocations of Evaluate. (Most of the work is done by the auxiliary functionInnerModuleEvaluation.) It performs the following steps when called:

  1. Assert: This call to Evaluate is not happening at the same time as another call to Evaluate within thesurrounding agent.
  2. Assert: module.[[Status]] islinkedorevaluated.
  3. Let stack be a new emptyList.
  4. Let result beInnerModuleEvaluation(module, stack, 0).
  5. If result is anabrupt completion, then
    1. For eachCyclic Module Recordm of stack, do
      1. Assert: m.[[Status]] isevaluating.
      2. Set m.[[Status]] toevaluated.
      3. Set m.[[EvaluationError]] to result.
    2. Assert: module.[[Status]] isevaluatedand module.[[EvaluationError]] is result.
    3. Return result.
  6. Assert: module.[[Status]] isevaluatedand module.[[EvaluationError]] isundefined.
  7. Assert: stack is empty.
  8. Returnundefined.

16.2.1.5.2.1 InnerModuleEvaluation ( module, stack, index )

The abstract operation InnerModuleEvaluation takes arguments module (aModule Record), stack, and index (a non-negativeinteger). It is used by Evaluate to perform the actual evaluation process for module, as well as recursively on all other modules in the dependency graph. The stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used the same way as inInnerModuleLinking. It performs the following steps when called:

  1. If module is not aCyclic Module Record, then
    1. Perform ? module.Evaluate().
    2. Return index.
  2. If module.[[Status]] isevaluated, then
    1. If module.[[EvaluationError]] isundefined, return index.
    2. Otherwise, return module.[[EvaluationError]].
  3. If module.[[Status]] isevaluating, return index.
  4. Assert: module.[[Status]] islinked.
  5. Set module.[[Status]] toevaluating.
  6. Set module.[[DFSIndex]] to index.
  7. Set module.[[DFSAncestorIndex]] to index.
  8. Set index to index + 1.
  9. Append module to stack.
  10. For each String required of module.[[RequestedModules]], do
    1. Let requiredModule be ! HostResolveImportedModule(module, required).
    2. NOTE: Link must be completed successfully prior to invoking this method, so every requested module is guaranteed to resolve successfully.
    3. Set index to ? InnerModuleEvaluation(requiredModule, stack, index).
    4. If requiredModule is aCyclic Module Record, then
      1. Assert: requiredModule.[[Status]] is eitherevaluatingorevaluated.
      2. Assert: requiredModule.[[Status]] isevaluatingif and only if requiredModule is in stack.
      3. If requiredModule.[[Status]] isevaluating, then
        1. Set module.[[DFSAncestorIndex]] tomin(module.[[DFSAncestorIndex]], requiredModule.[[DFSAncestorIndex]]).
  11. Perform ? module.ExecuteModule().
  12. Assert: module occurs exactly once in stack.
  13. Assert: module.[[DFSAncestorIndex]] ≤ module.[[DFSIndex]].
  14. If module.[[DFSAncestorIndex]] = module.[[DFSIndex]], then
    1. Let done befalse.
    2. Repeat, while done isfalse,
      1. Let requiredModule be the last element in stack.
      2. Remove the last element of stack.
      3. Assert: requiredModule is aCyclic Module Record.
      4. Set requiredModule.[[Status]] toevaluated.
      5. If requiredModule and module are the sameModule Record, set done totrue.
  15. Return index.

16.2.1.5.3 Example Cyclic Module Record Graphs

This non-normative section gives a series of examples of the linking and evaluation of a few common module graphs, with a specific focus on how errors can occur.

First consider the following simple module graph:

Figure 2: A simple module graph
A module graph in which module A depends on module B, and module B depends on module C

Let's first assume that there are no error conditions. When ahostfirst calls A.Link(), this will complete successfully by assumption, and recursively link modules B and C as well, such that A.[[Status]] = B.[[Status]] = C.[[Status]] =linked. This preparatory step can be performed at any time. Later, when thehostis ready to incur any possible side effects of the modules, it can call A.Evaluate(), which will complete successfully (again by assumption), recursively having evaluated first C and then B. Each module's [[Status]] at this point will beevaluated.

Consider then cases involving linking errors. IfInnerModuleLinkingof C succeeds but, thereafter, fails for B, for example because it imports something that C does not provide, then the original A.Link() will fail, and both A and B's [[Status]] remainunlinked. C's [[Status]] has becomelinked, though.

Finally, consider a case involving evaluation errors. IfInnerModuleEvaluationof C succeeds but, thereafter, fails for B, for example because B contains code that throws an exception, then the original A.Evaluate() will fail. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will becomeevaluated. C will also becomeevaluatedbut, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the exception ensures that any time ahosttries to reuse A or B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to reuse Cyclic Module Records; similarly, hosts are not required to expose the exception objects thrown by these methods. However, the specification enables such uses.)

The difference here between linking and evaluation errors is due to how evaluation must be only performed once, as it can cause side effects; it is thus important to remember whether evaluation has already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.) Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a later time with no issues.

Now consider a different type of error condition:

Figure 3: A module graph with an unresolvable module
A module graph in which module A depends on a missing (unresolvable) module, represented by ???

In this scenario, module A declares a dependency on some other module, but noModule Recordexists for that module, i.e.HostResolveImportedModulethrows an exception when asked for it. This could occur for a variety of reasons, such as the corresponding resource not existing, or the resource existing butParseModulethrowing an exception when trying to parse the resulting source text. Hosts can choose to expose the cause of failure via the exception they throw fromHostResolveImportedModule. In any case, this exception causes a linking failure, which as before results in A's [[Status]] remainingunlinked.

Lastly, consider a module graph with a cycle:

Figure 4: A cyclic module graph
A module graph in which module A depends on module B and C, but module B also depends on module A

Here we assume that the entry point is module A, so that thehostproceeds by calling A.Link(), which performsInnerModuleLinkingon A. This in turn callsInnerModuleLinkingon B. Because of the cycle, this again triggersInnerModuleLinkingon A, but at this point it is a no-op since A.[[Status]] is alreadylinking. B.[[Status]] itself remainslinkingwhen control gets back to A andInnerModuleLinkingis triggered on C. After this returns with C.[[Status]] beinglinked, both A and B transition fromlinkingtolinkedtogether; this is by design, since they form a strongly connected component.

An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.

Now consider a case where A has an linking error; for example, it tries to import a binding from C that does not exist. In that case, the above steps still occur, including the early return from the second call toInnerModuleLinkingon A. However, once we unwind back to the originalInnerModuleLinkingon A, it fails during InitializeEnvironment, namely right after C.ResolveExport(). The thrownSyntaxErrorexception propagates up to A.Link, which resets all modules that are currently on its stack (these are always exactly the modules that are stilllinking). Hence both A and B becomeunlinked. Note that C is left aslinked.

Finally, consider a case where A has an evaluation error; for example, its source code throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including the early return from the second call toInnerModuleEvaluationon A. However, once we unwind back to the originalInnerModuleEvaluationon A, it fails by assumption. The exception thrown propagates up to A.Evaluate(), which records the error in all modules that are currently on its stack (i.e., the modules that are stillevaluating). Hence both A and B becomeevaluatedand the exception is recorded in both A and B's [[EvaluationError]] fields, while C is left asevaluatedwith no [[EvaluationError]].

16.2.1.6 Source Text Module Records

A Source Text Module Record is used to represent information about a module that was defined from ECMAScript source text (11) that was parsed using thegoal symbolModule. Its fields contain digested information about the names that are imported by the module and its concrete methods use this digest to link, link, and evaluate the module.

ASource Text Module Recordcan exist in a module graph with other subclasses of the abstractModule Recordtype, and can participate in cycles with other subclasses of theCyclic Module Recordtype.

In addition to the fields defined inTable 46, Source Text Module Records have the additional fields listed inTable 48. Each of these fields is initially set inParseModule.

Table 48: Additional Fields of Source Text Module Records
Field NameValue TypeMeaning
[[ECMAScriptCode]]aParse NodeThe result of parsing the source text of this module usingModuleas thegoal symbol.
[[Context]]An ECMAScriptexecution context.Theexecution contextassociated with this module.
[[ImportMeta]]ObjectAn object exposed through the import.meta meta property. It isemptyuntil it is accessed by ECMAScript code.
[[ImportEntries]]Listof ImportEntry RecordsAListof ImportEntry records derived from the code of this module.
[[LocalExportEntries]]Listof ExportEntry RecordsAListof ExportEntry records derived from the code of this module that correspond to declarations that occur within the module.
[[IndirectExportEntries]]Listof ExportEntry RecordsAListof ExportEntry records derived from the code of this module that correspond to reexported imports that occur within the module or exports from export * as namespace declarations.
[[StarExportEntries]]Listof ExportEntry RecordsAListof ExportEntry records derived from the code of this module that correspond to export * declarations that occur within the module, not including export * as namespace declarations.

An ImportEntry Record is aRecordthat digests information about a single declarative import. EachImportEntry Recordhas the fields defined inTable 49:

Table 49:ImportEntry RecordFields
Field NameValue TypeMeaning
[[ModuleRequest]]StringString value of theModuleSpecifierof theImportDeclaration.
[[ImportName]]StringThe name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value"*"indicates that the import request is for the target module's namespace object.
[[LocalName]]StringThe name that is used to locally access the imported value from within the importing module.
Note 1

Table 50gives examples of ImportEntry records fields used to represent the syntactic import forms:

Table 50 (Informative): Import Forms Mappings to ImportEntry Records
Import Statement Form[[ModuleRequest]][[ImportName]][[LocalName]]
import v from "mod";"mod""default""v"
import * as ns from "mod";"mod""*""ns"
import {x} from "mod";"mod""x""x"
import {x as v} from "mod";"mod""x""v"
import "mod";AnImportEntry Recordis not created.

An ExportEntry Record is aRecordthat digests information about a single declarative export. EachExportEntry Recordhas the fields defined inTable 51:

Table 51:ExportEntry RecordFields
Field NameValue TypeMeaning
[[ExportName]]String | nullThe name used to export this binding by this module.
[[ModuleRequest]]String | nullThe String value of theModuleSpecifierof theExportDeclaration.nullif theExportDeclarationdoes not have aModuleSpecifier.
[[ImportName]]String | nullThe name under which the desired binding is exported by the module identified by [[ModuleRequest]].nullif theExportDeclarationdoes not have aModuleSpecifier."*"indicates that the export request is for all exported bindings.
[[LocalName]]String | nullThe name that is used to locally access the exported value from within the importing module.nullif the exported value is not locally accessible from within the module.
Note 2

Table 52gives examples of the ExportEntry record fields used to represent the syntactic export forms:

Table 52 (Informative): Export Forms Mappings to ExportEntry Records
Export Statement Form[[ExportName]][[ModuleRequest]][[ImportName]][[LocalName]]
export var v;"v"nullnull"v"
export default function f() {}"default"nullnull"f"
export default function () {}"default"nullnull"*default*"
export default 42;"default"nullnull"*default*"
export {x};"x"nullnull"x"
export {v as x};"x"nullnull"v"
export {x} from "mod";"x""mod""x"null
export {v as x} from "mod";"x""mod""v"null
export * from "mod";null"mod""*"null
export * as ns from "mod";"ns""mod""*"null

The following definitions specify the required concrete methods and otherabstract operationsfor Source Text Module Records

16.2.1.6.1 ParseModule ( sourceText, realm, hostDefined )

The abstract operation ParseModule takes arguments sourceText (ECMAScript source text), realm, and hostDefined. It creates aSource Text Module Recordbased upon the result of parsing sourceText as aModule. It performs the following steps when called:

  1. Assert: sourceText is an ECMAScript source text (see clause11).
  2. Let body beParseText(sourceText,Module).
  3. If body is aListof errors, return body.
  4. Let requestedModules be theModuleRequestsof body.
  5. Let importEntries beImportEntriesof body.
  6. Let importedBoundNames beImportedLocalNames(importEntries).
  7. Let indirectExportEntries be a new emptyList.
  8. Let localExportEntries be a new emptyList.
  9. Let starExportEntries be a new emptyList.
  10. Let exportEntries beExportEntriesof body.
  11. For eachExportEntry Recordee of exportEntries, do
    1. If ee.[[ModuleRequest]] isnull, then
      1. If ee.[[LocalName]] is not an element of importedBoundNames, then
        1. Append ee to localExportEntries.
      2. Else,
        1. Let ie be the element of importEntries whose [[LocalName]] is the same as ee.[[LocalName]].
        2. If ie.[[ImportName]] is"*", then
          1. NOTE: This is a re-export of an imported module namespace object.
          2. Append ee to localExportEntries.
        3. Else,
          1. NOTE: This is a re-export of a single name.
          2. Append theExportEntry Record{ [[ModuleRequest]]: ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]:null, [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
    2. Else if ee.[[ImportName]] is"*"and ee.[[ExportName]] isnull, then
      1. Append ee to starExportEntries.
    3. Else,
      1. Append ee to indirectExportEntries.
  12. ReturnSource Text Module Record{ [[Realm]]: realm, [[Environment]]:undefined, [[Namespace]]:undefined, [[Status]]:unlinked, [[EvaluationError]]:undefined, [[HostDefined]]: hostDefined, [[ECMAScriptCode]]: body, [[Context]]:empty, [[ImportMeta]]:empty, [[RequestedModules]]: requestedModules, [[ImportEntries]]: importEntries, [[LocalExportEntries]]: localExportEntries, [[IndirectExportEntries]]: indirectExportEntries, [[StarExportEntries]]: starExportEntries, [[DFSIndex]]:undefined, [[DFSAncestorIndex]]:undefined}.
Note

An implementation may parse module source text and analyse it for Early Error conditions prior to the evaluation of ParseModule for that module source text. However, the reporting of any errors must be deferred until the point where this specification actually performs ParseModule upon that source text.

16.2.1.6.2 GetExportedNames ( [ exportStarSet ] )

The GetExportedNames concrete method of aSource Text Module Recordmodule takes optional argument exportStarSet. It performs the following steps when called:

  1. If exportStarSet is not present, set exportStarSet to a new emptyList.
  2. Assert: exportStarSet is aListof Source Text Module Records.
  3. If exportStarSet contains module, then
    1. Assert: We've reached the starting point of an export * circularity.
    2. Return a new emptyList.
  4. Append module to exportStarSet.
  5. Let exportedNames be a new emptyList.
  6. For eachExportEntry Recorde of module.[[LocalExportEntries]], do
    1. Assert: module provides the direct binding for this export.
    2. Append e.[[ExportName]] to exportedNames.
  7. For eachExportEntry Recorde of module.[[IndirectExportEntries]], do
    1. Assert: module imports a specific binding for this export.
    2. Append e.[[ExportName]] to exportedNames.
  8. For eachExportEntry Recorde of module.[[StarExportEntries]], do
    1. Let requestedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
    2. Let starNames be ? requestedModule.GetExportedNames(exportStarSet).
    3. For each element n of starNames, do
      1. IfSameValue(n,"default") isfalse, then
        1. If n is not an element of exportedNames, then
          1. Append n to exportedNames.
  9. Return exportedNames.
Note

GetExportedNames does not filter out or throw an exception for names that have ambiguous star export bindings.

16.2.1.6.3 ResolveExport ( exportName [ , resolveSet ] )

The ResolveExport concrete method of aSource Text Module Recordmodule takes argument exportName (a String) and optional argument resolveSet.

ResolveExport attempts to resolve an imported binding to the actual defining module and local binding name. The defining module may be the module represented by theModule Recordthis method was invoked on or some other module that is imported by that module. The parameter resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of specificModule Recordand exportName is reached that is already in resolveSet, an import circularity has been encountered. Before recursively calling ResolveExport, a pair consisting of module and exportName is added to resolveSet.

If a defining module is found, aResolvedBinding Record{ [[Module]], [[BindingName]] } is returned. This record identifies the resolved binding of the originally requested export, unless this is the export of a namespace with no local binding. In this case, [[BindingName]] will be set to"*namespace*". If no definition was found or the request is found to be circular,nullis returned. If the request is found to be ambiguous, the string"ambiguous"is returned.

It performs the following steps when called:

  1. If resolveSet is not present, set resolveSet to a new emptyList.
  2. Assert: resolveSet is aListofRecord{ [[Module]], [[ExportName]] }.
  3. For eachRecord{ [[Module]], [[ExportName]] } r of resolveSet, do
    1. If module and r.[[Module]] are the sameModule RecordandSameValue(exportName, r.[[ExportName]]) istrue, then
      1. Assert: This is a circular import request.
      2. Returnnull.
  4. Append theRecord{ [[Module]]: module, [[ExportName]]: exportName } to resolveSet.
  5. For eachExportEntry Recorde of module.[[LocalExportEntries]], do
    1. IfSameValue(exportName, e.[[ExportName]]) istrue, then
      1. Assert: module provides the direct binding for this export.
      2. ReturnResolvedBinding Record{ [[Module]]: module, [[BindingName]]: e.[[LocalName]] }.
  6. For eachExportEntry Recorde of module.[[IndirectExportEntries]], do
    1. IfSameValue(exportName, e.[[ExportName]]) istrue, then
      1. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
      2. If e.[[ImportName]] is"*", then
        1. Assert: module does not provide the direct binding for this export.
        2. ReturnResolvedBinding Record{ [[Module]]: importedModule, [[BindingName]]:"*namespace*"}.
      3. Else,
        1. Assert: module imports a specific binding for this export.
        2. Return importedModule.ResolveExport(e.[[ImportName]], resolveSet).
  7. IfSameValue(exportName,"default") istrue, then
    1. Assert: A default export was not explicitly defined by this module.
    2. Returnnull.
    3. NOTE: A default export cannot be provided by an export * or export * from "mod" declaration.
  8. Let starResolution benull.
  9. For eachExportEntry Recorde of module.[[StarExportEntries]], do
    1. Let importedModule be ? HostResolveImportedModule(module, e.[[ModuleRequest]]).
    2. Let resolution be ? importedModule.ResolveExport(exportName, resolveSet).
    3. If resolution is"ambiguous", return"ambiguous".
    4. If resolution is notnull, then
      1. Assert: resolution is aResolvedBinding Record.
      2. If starResolution isnull, set starResolution to resolution.
      3. Else,
        1. Assert: There is more than one * import that includes the requested name.
        2. If resolution.[[Module]] and starResolution.[[Module]] are not the sameModule RecordorSameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) isfalse, return"ambiguous".
  10. Return starResolution.

16.2.1.6.4 InitializeEnvironment ( )

The InitializeEnvironment concrete method of aSource Text Module Recordmodule takes no arguments. It performs the following steps when called:

  1. For eachExportEntry Recorde of module.[[IndirectExportEntries]], do
    1. Let resolution be ? module.ResolveExport(e.[[ExportName]]).
    2. If resolution isnullor"ambiguous", throw aSyntaxErrorexception.
    3. Assert: resolution is aResolvedBinding Record.
  2. Assert: All named exports from module are resolvable.
  3. Let realm be module.[[Realm]].
  4. Assert: realm is notundefined.
  5. Let env beNewModuleEnvironment(realm.[[GlobalEnv]]).
  6. Set module.[[Environment]] to env.
  7. For eachImportEntry Recordin of module.[[ImportEntries]], do
    1. Let importedModule be ! HostResolveImportedModule(module, in.[[ModuleRequest]]).
    2. NOTE: The above call cannot fail because imported module requests are a subset of module.[[RequestedModules]], and these have been resolved earlier in this algorithm.
    3. If in.[[ImportName]] is"*", then
      1. Let namespace be ? GetModuleNamespace(importedModule).
      2. Perform ! env.CreateImmutableBinding(in.[[LocalName]],true).
      3. Call env.InitializeBinding(in.[[LocalName]], namespace).
    4. Else,
      1. Let resolution be ? importedModule.ResolveExport(in.[[ImportName]]).
      2. If resolution isnullor"ambiguous", throw aSyntaxErrorexception.
      3. If resolution.[[BindingName]] is"*namespace*", then
        1. Let namespace be ? GetModuleNamespace(resolution.[[Module]]).
        2. Perform ! env.CreateImmutableBinding(in.[[LocalName]],true).
        3. Call env.InitializeBinding(in.[[LocalName]], namespace).
      4. Else,
        1. Call env.CreateImportBinding(in.[[LocalName]], resolution.[[Module]], resolution.[[BindingName]]).
  8. Let moduleContext be a new ECMAScript codeexecution context.
  9. Set the Function of moduleContext tonull.
  10. Assert: module.[[Realm]] is notundefined.
  11. Set theRealmof moduleContext to module.[[Realm]].
  12. Set the ScriptOrModule of moduleContext to module.
  13. Set the VariableEnvironment of moduleContext to module.[[Environment]].
  14. Set the LexicalEnvironment of moduleContext to module.[[Environment]].
  15. Set the PrivateEnvironment of moduleContext tonull.
  16. Set module.[[Context]] to moduleContext.
  17. Push moduleContext onto theexecution context stack; moduleContext is now therunning execution context.
  18. Let code be module.[[ECMAScriptCode]].
  19. Let varDeclarations be theVarScopedDeclarationsof code.
  20. Let declaredVarNames be a new emptyList.
  21. For each element d of varDeclarations, do
    1. For each element dn of theBoundNamesof d, do
      1. If dn is not an element of declaredVarNames, then
        1. Perform ! env.CreateMutableBinding(dn,false).
        2. Call env.InitializeBinding(dn,undefined).
        3. Append dn to declaredVarNames.
  22. Let lexDeclarations be theLexicallyScopedDeclarationsof code.
  23. Let privateEnv benull.
  24. For each element d of lexDeclarations, do
    1. For each element dn of theBoundNamesof d, do
      1. IfIsConstantDeclarationof d istrue, then
        1. Perform ! env.CreateImmutableBinding(dn,true).
      2. Else,
        1. Perform ! env.CreateMutableBinding(dn,false).
      3. If d is aFunctionDeclaration, aGeneratorDeclaration, anAsyncFunctionDeclaration, or anAsyncGeneratorDeclaration, then
        1. Let fo beInstantiateFunctionObjectof d with arguments env and privateEnv.
        2. Call env.InitializeBinding(dn, fo).
  25. Remove moduleContext from theexecution context stack.
  26. ReturnNormalCompletion(empty).

16.2.1.6.5 ExecuteModule ( )

The ExecuteModule concrete method of aSource Text Module Recordmodule takes no arguments. It performs the following steps when called:

  1. Suspend the currentlyrunning execution context.
  2. Let moduleContext be module.[[Context]].
  3. Push moduleContext onto theexecution context stack; moduleContext is now therunning execution context.
  4. Let result be the result of evaluating module.[[ECMAScriptCode]].
  5. Suspend moduleContext and remove it from theexecution context stack.
  6. Resume the context that is now on the top of theexecution context stackas therunning execution context.
  7. ReturnCompletion(result).

16.2.1.7 HostResolveImportedModule ( referencingScriptOrModule, specifier )

Thehost-definedabstract operation HostResolveImportedModule takes arguments referencingScriptOrModule (aScript RecordorModule Recordornull) and specifier (aModuleSpecifierString). It provides the concreteModule Recordsubclass instance that corresponds to specifier occurring within the context of the script or module represented by referencingScriptOrModule. referencingScriptOrModule may benullif the resolution is being performed in the context of animport()expression and there is noactive script or moduleat that time.

Note

An example of when referencingScriptOrModule can benullis in a web browserhost. There, if a user clicks on a control given by

<button type="button" onclick="import('./foo.mjs')">Click me</button>

there will be noactive script or moduleat the time theimport()expression runs. More generally, this can happen in any situation where thehostpushes execution contexts withnullScriptOrModule components onto theexecution context stack.

An implementation of HostResolveImportedModule must conform to the following requirements:

  • If it completes normally, the [[Value]] slot of the completion must contain an instance of a concrete subclass ofModule Record.
  • If aModule Recordcorresponding to the pair referencingScriptOrModule, specifier does not exist or cannot be created, an exception must be thrown.
  • Each time this operation is called with a specific referencingScriptOrModule, specifier pair as arguments it must return the sameModule Recordinstance if it completes normally.

Multiple different referencingScriptOrModule, specifier pairs may map to the sameModule Recordinstance. The actual mapping semantic ishost-definedbut typically a normalization process is applied to specifier as part of the mapping process. A typical normalization process would include actions such as alphabetic case folding and expansion of relative and abbreviated path specifiers.

16.2.1.8 HostImportModuleDynamically ( referencingScriptOrModule, specifier, promiseCapability )

Thehost-definedabstract operation HostImportModuleDynamically takes arguments referencingScriptOrModule (aScript RecordorModule Recordornull), specifier (aModuleSpecifierString), and promiseCapability (aPromiseCapability Record). It performs any necessary setup work in order to make available the module corresponding to specifier occurring within the context of the script or module represented by referencingScriptOrModule. referencingScriptOrModule may benullif there is noactive script or modulewhen theimport()expression occurs. It then performsFinishDynamicImportto finish the dynamic import process.

An implementation of HostImportModuleDynamically must conform to the following requirements:

The actual process performed ishost-defined, but typically consists of performing whatever I/O operations are necessary to allowHostResolveImportedModuleto synchronously retrieve the appropriateModule Record, and then calling its Evaluate concrete method. This might require performing similar normalization asHostResolveImportedModuledoes.

16.2.1.9 FinishDynamicImport ( referencingScriptOrModule, specifier, promiseCapability, completion )

The abstract operation FinishDynamicImport takes arguments referencingScriptOrModule, specifier, promiseCapability (aPromiseCapability Record), and completion. FinishDynamicImport completes the process of a dynamic import originally started by animport()call, resolving or rejecting the promise returned by that call as appropriate according to completion. It is performed byhostenvironments as part ofHostImportModuleDynamically. It performs the following steps when called:

  1. If completion is anabrupt completion, perform ! Call(promiseCapability.[[Reject]],undefined, « completion.[[Value]] »).
  2. Else,
    1. Assert: completion is anormal completionand completion.[[Value]] isundefined.
    2. Let moduleRecord be ! HostResolveImportedModule(referencingScriptOrModule, specifier).
    3. Assert: Evaluate has already been invoked on moduleRecord and successfully completed.
    4. Let namespace beGetModuleNamespace(moduleRecord).
    5. If namespace is anabrupt completion, perform ! Call(promiseCapability.[[Reject]],undefined, « namespace.[[Value]] »).
    6. Else, perform ! Call(promiseCapability.[[Resolve]],undefined, « namespace.[[Value]] »).

16.2.1.10 GetModuleNamespace ( module )

The abstract operation GetModuleNamespace takes argument module. It retrieves the Module Namespace Object representing module's exports, lazily creating it the first time it was requested, and storing it in module.[[Namespace]] for future retrieval. It performs the following steps when called:

  1. Assert: module is an instance of a concrete subclass ofModule Record.
  2. Assert: If module is aCyclic Module Record, then module.[[Status]] is notunlinked.
  3. Let namespace be module.[[Namespace]].
  4. If namespace isundefined, then
    1. Let exportedNames be ? module.GetExportedNames().
    2. Let unambiguousNames be a new emptyList.
    3. For each element name of exportedNames, do
      1. Let resolution be ? module.ResolveExport(name).
      2. If resolution is aResolvedBinding Record, append name to unambiguousNames.
    4. Set namespace toModuleNamespaceCreate(module, unambiguousNames).
  5. Return namespace.
Note

The only way GetModuleNamespace can throw is via one of the triggeredHostResolveImportedModulecalls. Unresolvable names are simply excluded from the namespace at this point. They will lead to a real linking error later unless they are all ambiguous star exports that are not explicitly requested anywhere.

16.2.1.11 Runtime Semantics: Evaluation

Module:[empty]
  1. ReturnNormalCompletion(undefined).
ModuleBody:ModuleItemList
  1. Let result be the result of evaluatingModuleItemList.
  2. If result.[[Type]] isnormaland result.[[Value]] isempty, then
    1. ReturnNormalCompletion(undefined).
  3. ReturnCompletion(result).
ModuleItemList:ModuleItemListModuleItem
  1. Let sl be the result of evaluatingModuleItemList.
  2. ReturnIfAbrupt(sl).
  3. Let s be the result of evaluatingModuleItem.
  4. ReturnCompletion(UpdateEmpty(s, sl)).
Note

The value of aModuleItemListis the value of the last value-producing item in theModuleItemList.

ModuleItem:ImportDeclaration
  1. ReturnNormalCompletion(empty).

16.2.2 Imports

Syntax

ImportDeclaration:importImportClauseFromClause;importModuleSpecifier;ImportClause:ImportedDefaultBindingNameSpaceImportNamedImportsImportedDefaultBinding,NameSpaceImportImportedDefaultBinding,NamedImportsImportedDefaultBinding:ImportedBindingNameSpaceImport:*asImportedBindingNamedImports:{}{ImportsList}{ImportsList,}FromClause:fromModuleSpecifierImportsList:ImportSpecifierImportsList,ImportSpecifierImportSpecifier:ImportedBindingIdentifierNameasImportedBindingModuleSpecifier:StringLiteralImportedBinding:BindingIdentifier[~Yield, ~Await]

16.2.2.1 Static Semantics: Early Errors

ModuleItem:ImportDeclaration

16.2.2.2 Static Semantics: ImportEntries

Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItemListModuleItem
  1. Let entries1 beImportEntriesofModuleItemList.
  2. Let entries2 beImportEntriesofModuleItem.
  3. Return thelist-concatenationof entries1 and entries2.
ModuleItem:ExportDeclarationStatementListItem
  1. Return a new emptyList.
ImportDeclaration:importImportClauseFromClause;
  1. Let module be the sole element ofModuleRequestsofFromClause.
  2. ReturnImportEntriesForModuleofImportClausewith argument module.
ImportDeclaration:importModuleSpecifier;
  1. Return a new emptyList.

16.2.2.3 Static Semantics: ImportEntriesForModule

With parameter module.

ImportClause:ImportedDefaultBinding,NameSpaceImport
  1. Let entries1 beImportEntriesForModuleofImportedDefaultBindingwith argument module.
  2. Let entries2 beImportEntriesForModuleofNameSpaceImportwith argument module.
  3. Return thelist-concatenationof entries1 and entries2.
ImportClause:ImportedDefaultBinding,NamedImports
  1. Let entries1 beImportEntriesForModuleofImportedDefaultBindingwith argument module.
  2. Let entries2 beImportEntriesForModuleofNamedImportswith argument module.
  3. Return thelist-concatenationof entries1 and entries2.
ImportedDefaultBinding:ImportedBinding
  1. Let localName be the sole element ofBoundNamesofImportedBinding.
  2. Let defaultEntry be theImportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]:"default", [[LocalName]]: localName }.
  3. Return aListwhose sole element is defaultEntry.
NameSpaceImport:*asImportedBinding
  1. Let localName be theStringValueofImportedBinding.
  2. Let entry be theImportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]:"*", [[LocalName]]: localName }.
  3. Return aListwhose sole element is entry.
NamedImports:{}
  1. Return a new emptyList.
ImportsList:ImportsList,ImportSpecifier
  1. Let specs1 be theImportEntriesForModuleofImportsListwith argument module.
  2. Let specs2 be theImportEntriesForModuleofImportSpecifierwith argument module.
  3. Return thelist-concatenationof specs1 and specs2.
ImportSpecifier:ImportedBinding
  1. Let localName be the sole element ofBoundNamesofImportedBinding.
  2. Let entry be theImportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]: localName, [[LocalName]]: localName }.
  3. Return aListwhose sole element is entry.
ImportSpecifier:IdentifierNameasImportedBinding
  1. Let importName be theStringValueofIdentifierName.
  2. Let localName be theStringValueofImportedBinding.
  3. Let entry be theImportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName }.
  4. Return aListwhose sole element is entry.

16.2.3 Exports

Syntax

ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;exportVariableStatement[~Yield, ~Await]exportDeclaration[~Yield, ~Await]exportdefaultHoistableDeclaration[~Yield, ~Await, +Default]exportdefaultClassDeclaration[~Yield, ~Await, +Default]exportdefault[lookahead ∉ {function,async[noLineTerminatorhere]function,class}]AssignmentExpression[+In, ~Yield, ~Await];ExportFromClause:**asIdentifierNameNamedExportsNamedExports:{}{ExportsList}{ExportsList,}ExportsList:ExportSpecifierExportsList,ExportSpecifierExportSpecifier:IdentifierNameIdentifierNameasIdentifierName

16.2.3.1 Static Semantics: Early Errors

ExportDeclaration:exportNamedExports;Note

The above rule means that eachReferencedBindingsofNamedExportsis treated as anIdentifierReference.

16.2.3.2 Static Semantics: ExportedBindings

Note

ExportedBindings are the locally bound names that are explicitly associated with aModule'sExportedNames.

ModuleItemList:ModuleItemListModuleItem
  1. Let names1 beExportedBindingsofModuleItemList.
  2. Let names2 beExportedBindingsofModuleItem.
  3. Return thelist-concatenationof names1 and names2.
ModuleItem:ImportDeclarationStatementListItem
  1. Return a new emptyList.
ExportDeclaration:exportExportFromClauseFromClause;
  1. Return a new emptyList.
ExportDeclaration:exportNamedExports;
  1. Return theExportedBindingsofNamedExports.
ExportDeclaration:exportVariableStatement
  1. Return theBoundNamesofVariableStatement.
ExportDeclaration:exportDeclaration
  1. Return theBoundNamesofDeclaration.
ExportDeclaration:exportdefaultHoistableDeclarationexportdefaultClassDeclarationexportdefaultAssignmentExpression;
  1. Return theBoundNamesof thisExportDeclaration.
NamedExports:{}
  1. Return a new emptyList.
ExportsList:ExportsList,ExportSpecifier
  1. Let names1 be theExportedBindingsofExportsList.
  2. Let names2 be theExportedBindingsofExportSpecifier.
  3. Return thelist-concatenationof names1 and names2.
ExportSpecifier:IdentifierName
  1. Return aListwhose sole element is theStringValueofIdentifierName.
ExportSpecifier:IdentifierNameasIdentifierName
  1. Return aListwhose sole element is theStringValueof the firstIdentifierName.

16.2.3.3 Static Semantics: ExportedNames

Note

ExportedNames are the externally visible names that aModuleexplicitly maps to one of its local name bindings.

ModuleItemList:ModuleItemListModuleItem
  1. Let names1 beExportedNamesofModuleItemList.
  2. Let names2 beExportedNamesofModuleItem.
  3. Return thelist-concatenationof names1 and names2.
ModuleItem:ExportDeclaration
  1. Return theExportedNamesofExportDeclaration.
ModuleItem:ImportDeclarationStatementListItem
  1. Return a new emptyList.
ExportDeclaration:exportExportFromClauseFromClause;
  1. Return theExportedNamesofExportFromClause.
ExportFromClause:*
  1. Return a new emptyList.
ExportFromClause:*asIdentifierName
  1. Return aListwhose sole element is theStringValueofIdentifierName.
ExportFromClause:NamedExports
  1. Return theExportedNamesofNamedExports.
ExportDeclaration:exportVariableStatement
  1. Return theBoundNamesofVariableStatement.
ExportDeclaration:exportDeclaration
  1. Return theBoundNamesofDeclaration.
ExportDeclaration:exportdefaultHoistableDeclarationexportdefaultClassDeclarationexportdefaultAssignmentExpression;
  1. Return «"default"».
NamedExports:{}
  1. Return a new emptyList.
ExportsList:ExportsList,ExportSpecifier
  1. Let names1 be theExportedNamesofExportsList.
  2. Let names2 be theExportedNamesofExportSpecifier.
  3. Return thelist-concatenationof names1 and names2.
ExportSpecifier:IdentifierName
  1. Return aListwhose sole element is theStringValueofIdentifierName.
ExportSpecifier:IdentifierNameasIdentifierName
  1. Return aListwhose sole element is theStringValueof the secondIdentifierName.

16.2.3.4 Static Semantics: ExportEntries

Module:[empty]
  1. Return a new emptyList.
ModuleItemList:ModuleItemListModuleItem
  1. Let entries1 beExportEntriesofModuleItemList.
  2. Let entries2 beExportEntriesofModuleItem.
  3. Return thelist-concatenationof entries1 and entries2.
ModuleItem:ImportDeclarationStatementListItem
  1. Return a new emptyList.
ExportDeclaration:exportExportFromClauseFromClause;
  1. Let module be the sole element ofModuleRequestsofFromClause.
  2. ReturnExportEntriesForModuleofExportFromClausewith argument module.
ExportDeclaration:exportNamedExports;
  1. ReturnExportEntriesForModuleofNamedExportswith argumentnull.
ExportDeclaration:exportVariableStatement
  1. Let entries be a new emptyList.
  2. Let names be theBoundNamesofVariableStatement.
  3. For each element name of names, do
    1. Append theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: name, [[ExportName]]: name } to entries.
  4. Return entries.
ExportDeclaration:exportDeclaration
  1. Let entries be a new emptyList.
  2. Let names be theBoundNamesofDeclaration.
  3. For each element name of names, do
    1. Append theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: name, [[ExportName]]: name } to entries.
  4. Return entries.
ExportDeclaration:exportdefaultHoistableDeclaration
  1. Let names beBoundNamesofHoistableDeclaration.
  2. Let localName be the sole element of names.
  3. Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: localName, [[ExportName]]:"default"}.
ExportDeclaration:exportdefaultClassDeclaration
  1. Let names beBoundNamesofClassDeclaration.
  2. Let localName be the sole element of names.
  3. Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]: localName, [[ExportName]]:"default"}.
ExportDeclaration:exportdefaultAssignmentExpression;
  1. Let entry be theExportEntry Record{ [[ModuleRequest]]:null, [[ImportName]]:null, [[LocalName]]:"*default*", [[ExportName]]:"default"}.
  2. Return aListwhose sole element is entry.
Note

"*default*"is used within this specification as a synthetic name for anonymous default export values.

16.2.3.5 Static Semantics: ExportEntriesForModule

With parameter module.

ExportFromClause:*
  1. Let entry be theExportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]:"*", [[LocalName]]:null, [[ExportName]]:null}.
  2. Return aListwhose sole element is entry.
ExportFromClause:*asIdentifierName
  1. Let exportName be theStringValueofIdentifierName.
  2. Let entry be theExportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]:"*", [[LocalName]]:null, [[ExportName]]: exportName }.
  3. Return aListwhose sole element is entry.
NamedExports:{}
  1. Return a new emptyList.
ExportsList:ExportsList,ExportSpecifier
  1. Let specs1 be theExportEntriesForModuleofExportsListwith argument module.
  2. Let specs2 be theExportEntriesForModuleofExportSpecifierwith argument module.
  3. Return thelist-concatenationof specs1 and specs2.
ExportSpecifier:IdentifierName
  1. Let sourceName be theStringValueofIdentifierName.
  2. If module isnull, then
    1. Let localName be sourceName.
    2. Let importName benull.
  3. Else,
    1. Let localName benull.
    2. Let importName be sourceName.
  4. Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
ExportSpecifier:IdentifierNameasIdentifierName
  1. Let sourceName be theStringValueof the firstIdentifierName.
  2. Let exportName be theStringValueof the secondIdentifierName.
  3. If module isnull, then
    1. Let localName be sourceName.
    2. Let importName benull.
  4. Else,
    1. Let localName benull.
    2. Let importName be sourceName.
  5. Return aListwhose sole element is theExportEntry Record{ [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.

16.2.3.6 Static Semantics: ReferencedBindings

NamedExports:{}
  1. Return a new emptyList.
ExportsList:ExportsList,ExportSpecifier
  1. Let names1 be theReferencedBindingsofExportsList.
  2. Let names2 be theReferencedBindingsofExportSpecifier.
  3. Return thelist-concatenationof names1 and names2.
ExportSpecifier:IdentifierName
  1. Return aListwhose sole element is theIdentifierName.
ExportSpecifier:IdentifierNameasIdentifierName
  1. Return aListwhose sole element is the firstIdentifierName.

16.2.3.7 Runtime Semantics: Evaluation

ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;
  1. ReturnNormalCompletion(empty).
ExportDeclaration:exportVariableStatement
  1. Return the result of evaluatingVariableStatement.
ExportDeclaration:exportDeclaration
  1. Return the result of evaluatingDeclaration.
ExportDeclaration:exportdefaultHoistableDeclaration
  1. Return the result of evaluatingHoistableDeclaration.
ExportDeclaration:exportdefaultClassDeclaration
  1. Let value be ?BindingClassDeclarationEvaluationofClassDeclaration.
  2. Let className be the sole element ofBoundNamesofClassDeclaration.
  3. If className is"*default*", then
    1. Let env be therunning execution context's LexicalEnvironment.
    2. Perform ? InitializeBoundName("*default*", value, env).
  4. ReturnNormalCompletion(empty).
ExportDeclaration:exportdefaultAssignmentExpression;
  1. IfIsAnonymousFunctionDefinition(AssignmentExpression) istrue, then
    1. Let value be ?NamedEvaluationofAssignmentExpressionwith argument"default".
  2. Else,
    1. Let rhs be the result of evaluatingAssignmentExpression.
    2. Let value be ? GetValue(rhs).
  3. Let env be therunning execution context's LexicalEnvironment.
  4. Perform ? InitializeBoundName("*default*", value, env).
  5. ReturnNormalCompletion(empty).

17 Error Handling and Language Extensions

An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated. An early error is an error that can be detected and reported prior to the evaluation of any construct in theScriptcontaining the error. The presence of anearly errorprevents the evaluation of the construct. An implementation must report early errors in aScriptas part of parsing thatScriptinParseScript. Early errors in aModuleare reported at the point when theModulewould be evaluated and theModuleis never initialized. Early errors in eval code are reported at the time eval is called and prevent evaluation of the eval code. All errors that are not early errors are runtime errors.

An implementation must report as anearly errorany occurrence of a condition that is listed in a “Static Semantics: Early Errors” subclause of this specification.

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a construct cannot execute without error under any circumstances. An implementation may issue an early warning in such a case, but it should not report the error until the relevant construct is actually executed.

An implementation shall report all errors as specified, except for the following:

17.1 Forbidden Extensions

An implementation must not extend this specification in the following ways:

18 ECMAScript Standard Built-in Objects

There are certain built-in objects available whenever an ECMAScriptScriptorModulebegins execution. One, theglobal object, is part of the global environment of the executing program. Others are accessible as initial properties of theglobal objector indirectly as properties of accessible built-in objects.

Unless specified otherwise, a built-in object that is callable as a function is a built-infunction objectwith the characteristics described in10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the valuetrue. Every built-infunction objecthas a [[Realm]] internal slot whose value is theRealm Recordof therealmfor which the object was initially created.

Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended for use with the new operator. For each built-in function, this specification describes the arguments required by that function and the properties of thatfunction object. For each built-inconstructor, this specification furthermore describes properties of the prototype object of thatconstructorand properties of specific object instances returned by a new expression that invokes thatconstructor.

Unless otherwise specified in the description of a particular function, if a built-in function orconstructoris given fewer arguments than the function is specified to require, the function orconstructorshall behave exactly as if it had been given sufficient additional arguments, each such argument being theundefinedvalue. Such missing arguments are considered to be “not present” and may be identified in that manner by specification algorithms. In the description of a particular function, the terms “thisvalue” and “NewTarget” have the meanings given in10.3.

Unless otherwise specified in the description of a particular function, if a built-in function orconstructordescribed is given more arguments than the function is specified to allow, the extra arguments are evaluated by the call and then ignored by the function. However, an implementation may define implementation specific behaviour relating to such arguments as long as the behaviour is not the throwing of aTypeErrorexception that is predicated simply on the presence of an extra argument.

Note 1

Implementations that add additional capabilities to the set of built-in functions are encouraged to do so by adding new functions rather than adding new parameters to existing functions.

Unless otherwise specified every built-in function and every built-inconstructorhas theFunction prototype object, which is the initial value of the expression Function.prototype (20.2.3), as the value of its [[Prototype]] internal slot.

Unless otherwise specified every built-in prototype object has theObject prototype object, which is the initial value of the expression Object.prototype (20.1.3), as the value of its [[Prototype]] internal slot, except theObject prototype objectitself.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

Each built-in function defined in this specification is created by calling theCreateBuiltinFunctionabstract operation (10.3.3). The values of the length and name parameters are the initial values of the"length"and"name"properties as discussed below. The values of the prefix parameter are similarly discussed below.

Every built-infunction object, including constructors, has a"length"property whose value is a non-negativeintegral Number. Unless otherwise specified, this value is equal to the number of required parameters shown in the subclause heading for the function description. Optional parameters and rest parameters are not included in the parameter count.

Note 2

For example, thefunction objectthat is the initial value of the"map"property of theArray prototype objectis described under the subclause heading «Array.prototype.map (callbackFn [ , thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore the value of the"length"property of thatfunction objectis1𝔽.

Unless otherwise specified, the"length"property of a built-infunction objecthas the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

Every built-infunction object, including constructors, has a"name"property whose value is a String. Unless otherwise specified, this value is the name that is given to the function in this specification. Functions that are identified as anonymous functions use the empty String as the value of the"name"property. For functions that are specified as properties of objects, the name value is theproperty namestring used to access the function. Functions that are specified as get or set accessor functions of built-in properties have"get"or"set"(respectively) passed to the prefix parameter when callingCreateBuiltinFunction.

The value of the"name"property is explicitly specified for each built-in functions whose property key is a Symbol value. If such an explicitly specified value starts with the prefix"get "or"set "and the function for which it is specified is a get or set accessor function of a built-in property, the value without the prefix is passed to the name parameter, and the value"get"or"set"(respectively) is passed to the prefix parameter when callingCreateBuiltinFunction.

Unless otherwise specified, the"name"property of a built-infunction objecthas the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

Every otherdata propertydescribed in clauses19through28and in AnnexB.2has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true} unless otherwise specified.

Everyaccessor propertydescribed in clauses19through28and in AnnexB.2has the attributes { [[Enumerable]]:false, [[Configurable]]:true} unless otherwise specified. If only a get accessor function is described, the set accessor function is the default value,undefined. If only a set accessor is described the get accessor is the default value,undefined.

19 The Global Object

The global object:

19.1 Value Properties of the Global Object

19.1.1 globalThis

The initial value of the"globalThis"property of theglobal objectin aRealm Recordrealm is realm.[[GlobalEnv]].[[GlobalThisValue]].

This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.

19.1.2 Infinity

The value of Infinity is+∞𝔽 (see6.1.6.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

19.1.3 NaN

The value of NaN isNaN(see6.1.6.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

19.1.4 undefined

The value of undefined isundefined(see6.1.1). This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

19.2 Function Properties of the Global Object

19.2.1 eval ( x )

The eval function is the %eval% intrinsic object. When the eval function is called with one argument x, the following steps are taken:

  1. Assert: Theexecution context stackhas at least two elements.
  2. Let callerContext be the second to top element of theexecution context stack.
  3. Let callerRealm be callerContext'sRealm.
  4. Return ? PerformEval(x, callerRealm,false,false).

19.2.1.1 PerformEval ( x, callerRealm, strictCaller, direct )

The abstract operation PerformEval takes arguments x, callerRealm, strictCaller, and direct. It performs the following steps when called:

  1. Assert: If direct isfalse, then strictCaller is alsofalse.
  2. IfType(x) is not String, return x.
  3. Let evalRealm bethe current Realm Record.
  4. Perform ? HostEnsureCanCompileStrings(callerRealm, evalRealm).
  5. Let inFunction befalse.
  6. Let inMethod befalse.
  7. Let inDerivedConstructor befalse.
  8. Let inClassFieldInitializer befalse.
  9. If direct istrue, then
    1. Let thisEnvRec be ! GetThisEnvironment().
    2. If thisEnvRec is afunction Environment Record, then
      1. Let F be thisEnvRec.[[FunctionObject]].
      2. Set inFunction totrue.
      3. Set inMethod to thisEnvRec.HasSuperBinding().
      4. If F.[[ConstructorKind]] isderived, set inDerivedConstructor totrue.
      5. Let classFieldIntializerName be F.[[ClassFieldInitializerName]].
      6. If classFieldIntializerName is notempty, set inClassFieldInitializer totrue.
  10. Perform the following substeps in animplementation-definedorder, possibly interleaving parsing and error detection:
    1. Let script beParseText(!StringToCodePoints(x),Script).
    2. If script is aListof errors, throw aSyntaxErrorexception.
    3. If scriptContainsScriptBodyisfalse, returnundefined.
    4. Let body be theScriptBodyof script.
    5. If inFunction isfalse, and bodyContainsNewTarget, throw aSyntaxErrorexception.
    6. If inMethod isfalse, and bodyContainsSuperProperty, throw aSyntaxErrorexception.
    7. If inDerivedConstructor isfalse, and bodyContainsSuperCall, throw aSyntaxErrorexception.
    8. If inClassFieldInitializer istrue, andContainsArgumentsof body istrue, throw aSyntaxErrorexception.
  11. If strictCaller istrue, let strictEval betrue.
  12. Else, let strictEval beIsStrictof script.
  13. Let runningContext be therunning execution context.
  14. NOTE: If direct istrue, runningContext will be theexecution contextthat performed thedirect eval. If direct isfalse, runningContext will be theexecution contextfor the invocation of the eval function.
  15. If direct istrue, then
    1. Let lexEnv beNewDeclarativeEnvironment(runningContext's LexicalEnvironment).
    2. Let varEnv be runningContext's VariableEnvironment.
    3. Let privateEnv be runningContext's PrivateEnvironment.
  16. Else,
    1. Let lexEnv beNewDeclarativeEnvironment(evalRealm.[[GlobalEnv]]).
    2. Let varEnv be evalRealm.[[GlobalEnv]].
    3. Let privateEnv benull.
  17. If strictEval istrue, set varEnv to lexEnv.
  18. If runningContext is not already suspended, suspend runningContext.
  19. Let evalContext be a new ECMAScript codeexecution context.
  20. Set evalContext's Function tonull.
  21. Set evalContext'sRealmto evalRealm.
  22. Set evalContext's ScriptOrModule to runningContext's ScriptOrModule.
  23. Set evalContext's VariableEnvironment to varEnv.
  24. Set evalContext's LexicalEnvironment to lexEnv.
  25. Set evalContext's PrivateEnvironment to privateEnv.
  26. Push evalContext onto theexecution context stack; evalContext is now therunning execution context.
  27. Let result beEvalDeclarationInstantiation(body, varEnv, lexEnv, privateEnv, strictEval).
  28. If result.[[Type]] isnormal, then
    1. Set result to the result of evaluating body.
  29. If result.[[Type]] isnormaland result.[[Value]] isempty, then
    1. Set result toNormalCompletion(undefined).
  30. Suspend evalContext and remove it from theexecution context stack.
  31. Resume the context that is now on the top of theexecution context stackas therunning execution context.
  32. ReturnCompletion(result).
Note

The eval code cannot instantiate variable or function bindings in the variable environment of the calling context that invoked the eval if either the code of the calling context or the eval code isstrict mode code. Instead such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code. Bindings introduced by let, const, or class declarations are always instantiated in a new LexicalEnvironment.

19.2.1.2 HostEnsureCanCompileStrings ( callerRealm, calleeRealm )

Thehost-definedabstract operation HostEnsureCanCompileStrings takes arguments callerRealm (aRealm Record) and calleeRealm (aRealm Record). It allowshostenvironments to block certain ECMAScript functions which allow developers to compile strings into ECMAScript code.

An implementation of HostEnsureCanCompileStrings must conform to the following requirements:

The default implementation of HostEnsureCanCompileStrings is to returnNormalCompletion(empty).

19.2.1.3 EvalDeclarationInstantiation ( body, varEnv, lexEnv, privateEnv, strict )

The abstract operation EvalDeclarationInstantiation takes arguments body, varEnv, lexEnv, privateEnv, and strict. It performs the following steps when called:

  1. Let varNames be theVarDeclaredNamesof body.
  2. Let varDeclarations be theVarScopedDeclarationsof body.
  3. If strict isfalse, then
    1. If varEnv is aglobal Environment Record, then
      1. For each element name of varNames, do
        1. If varEnv.HasLexicalDeclaration(name) istrue, throw aSyntaxErrorexception.
        2. NOTE: eval will not create a global var declaration that would be shadowed by a global lexical declaration.
    2. Let thisEnv be lexEnv.
    3. Assert: The following loop will terminate.
    4. Repeat, while thisEnv is not the same as varEnv,
      1. If thisEnv is not anobject Environment Record, then
        1. NOTE: The environment of with statements cannot contain any lexical declaration so it doesn't need to be checked for var/let hoisting conflicts.
        2. For each element name of varNames, do
          1. If thisEnv.HasBinding(name) istrue, then
            1. Throw aSyntaxErrorexception.
            2. NOTE: AnnexB.3.4defines alternate semantics for the above step.
          2. NOTE: Adirect evalwill not hoist var declaration over a like-named lexical declaration.
      2. Set thisEnv to thisEnv.[[OuterEnv]].
  4. Let privateIdentifiers be a new emptyList.
  5. Let pointer be privateEnv.
  6. Repeat, while pointer is notnull,
    1. For eachPrivate Namebinding of pointer.[[Names]], do
      1. If privateIdentifiers does not contain binding.[[Description]], append binding.[[Description]] to privateIdentifiers.
    2. Set pointer to pointer.[[OuterPrivateEnvironment]].
  7. IfAllPrivateIdentifiersValidof body with argument privateIdentifiers isfalse, throw aSyntaxErrorexception.
  8. Let functionsToInitialize be a new emptyList.
  9. Let declaredFunctionNames be a new emptyList.
  10. For each element d of varDeclarations, in reverseListorder, do
    1. If d is neither aVariableDeclarationnor aForBindingnor aBindingIdentifier, then
      1. Assert: d is either aFunctionDeclaration, aGeneratorDeclaration, anAsyncFunctionDeclaration, or anAsyncGeneratorDeclaration.
      2. NOTE: If there are multiple function declarations for the same name, the last declaration is used.
      3. Let fn be the sole element of theBoundNamesof d.
      4. If fn is not an element of declaredFunctionNames, then
        1. If varEnv is aglobal Environment Record, then
          1. Let fnDefinable be ? varEnv.CanDeclareGlobalFunction(fn).
          2. If fnDefinable isfalse, throw aTypeErrorexception.
        2. Append fn to declaredFunctionNames.
        3. Insert d as the first element of functionsToInitialize.
  11. NOTE: AnnexB.3.2.3adds additional steps at this point.
  12. Let declaredVarNames be a new emptyList.
  13. For each element d of varDeclarations, do
    1. If d is aVariableDeclaration, aForBinding, or aBindingIdentifier, then
      1. For each String vn of theBoundNamesof d, do
        1. If vn is not an element of declaredFunctionNames, then
          1. If varEnv is aglobal Environment Record, then
            1. Let vnDefinable be ? varEnv.CanDeclareGlobalVar(vn).
            2. If vnDefinable isfalse, throw aTypeErrorexception.
          2. If vn is not an element of declaredVarNames, then
            1. Append vn to declaredVarNames.
  14. NOTE: No abnormal terminations occur after this algorithm step unless varEnv is aglobal Environment Recordand theglobal objectis aProxy exotic object.
  15. Let lexDeclarations be theLexicallyScopedDeclarationsof body.
  16. For each element d of lexDeclarations, do
    1. NOTE: Lexically declared names are only instantiated here but not initialized.
    2. For each element dn of theBoundNamesof d, do
      1. IfIsConstantDeclarationof d istrue, then
        1. Perform ? lexEnv.CreateImmutableBinding(dn,true).
      2. Else,
        1. Perform ? lexEnv.CreateMutableBinding(dn,false).
  17. For eachParse Nodef of functionsToInitialize, do
    1. Let fn be the sole element of theBoundNamesof f.
    2. Let fo beInstantiateFunctionObjectof f with arguments lexEnv and privateEnv.
    3. If varEnv is aglobal Environment Record, then
      1. Perform ? varEnv.CreateGlobalFunctionBinding(fn, fo,true).
    4. Else,
      1. Let bindingExists be varEnv.HasBinding(fn).
      2. If bindingExists isfalse, then
        1. Let status be ! varEnv.CreateMutableBinding(fn,true).
        2. Assert: status is not anabrupt completionbecause of validation preceding step14.
        3. Perform ! varEnv.InitializeBinding(fn, fo).
      3. Else,
        1. Perform ! varEnv.SetMutableBinding(fn, fo,false).
  18. For each String vn of declaredVarNames, do
    1. If varEnv is aglobal Environment Record, then
      1. Perform ? varEnv.CreateGlobalVarBinding(vn,true).
    2. Else,
      1. Let bindingExists be varEnv.HasBinding(vn).
      2. If bindingExists isfalse, then
        1. Let status be ! varEnv.CreateMutableBinding(vn,true).
        2. Assert: status is not anabrupt completionbecause of validation preceding step14.
        3. Perform ! varEnv.InitializeBinding(vn,undefined).
  19. ReturnNormalCompletion(empty).
Note

An alternative version of this algorithm is described inB.3.4.

19.2.2 isFinite ( number )

The isFinite function is the %isFinite% intrinsic object. When the isFinite function is called with one argument number, the following steps are taken:

  1. Let num be ? ToNumber(number).
  2. If num isNaN,+∞𝔽, or-∞𝔽, returnfalse.
  3. Otherwise, returntrue.

19.2.3 isNaN ( number )

The isNaN function is the %isNaN% intrinsic object. When the isNaN function is called with one argument number, the following steps are taken:

  1. Let num be ? ToNumber(number).
  2. If num isNaN, returntrue.
  3. Otherwise, returnfalse.
Note

A reliable way for ECMAScript code to test if a value X is aNaNis an expression of the form X !== X. The result will betrueif and only if X is aNaN.

19.2.4 parseFloat ( string )

The parseFloat function produces aNumber valuedictated by interpretation of the contents of the string argument as a decimal literal.

The parseFloat function is the %parseFloat% intrinsic object. When the parseFloat function is called with one argument string, the following steps are taken:

  1. Let inputString be ? ToString(string).
  2. Let trimmedString be ! TrimString(inputString,start).
  3. If neither trimmedString nor any prefix of trimmedString satisfies the syntax of aStrDecimalLiteral(see7.1.4.1), returnNaN.
  4. Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies the syntax of aStrDecimalLiteral.
  5. Let parsedNumber beParseText(!StringToCodePoints(numberString),StrDecimalLiteral).
  6. Assert: parsedNumber is aParse Node.
  7. ReturnStringNumericValueof parsedNumber.
Note

parseFloat may interpret only a leading portion of string as aNumber value; it ignores any code units that cannot be interpreted as part of the notation of a decimal literal, and no indication is given that any such code units were ignored.

19.2.5 parseInt ( string, radix )

The parseInt function produces anintegral Numberdictated by interpretation of the contents of the string argument according to the specified radix. Leading white space in string is ignored. If radix isundefinedor 0, it is assumed to be 10 except when the number begins with the code unit pairs 0x or 0X, in which case a radix of 16 is assumed. If radix is 16, the number may also optionally begin with the code unit pairs 0x or 0X.

The parseInt function is the %parseInt% intrinsic object. When the parseInt function is called, the following steps are taken:

  1. Let inputString be ? ToString(string).
  2. Let S be ! TrimString(inputString,start).
  3. Let sign be 1.
  4. If S is not empty and the first code unit of S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
  5. If S is not empty and the first code unit of S is the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), remove the first code unit from S.
  6. Let R be(?ToInt32(radix)).
  7. Let stripPrefix betrue.
  8. If R ≠ 0, then
    1. If R < 2 or R > 36, returnNaN.
    2. If R ≠ 16, set stripPrefix tofalse.
  9. Else,
    1. Set R to 10.
  10. If stripPrefix istrue, then
    1. If the length of S is at least 2 and the first two code units of S are either"0x"or"0X", then
      1. Remove the first two code units from S.
      2. Set R to 16.
  11. If S contains a code unit that is not a radix-R digit, let end be the index within S of the first such code unit; otherwise, let end be the length of S.
  12. Let Z be thesubstringof S from 0 to end.
  13. If Z is empty, returnNaN.
  14. Let mathInt be theintegervalue that is represented by Z in radix-R notation, using the letters A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20 significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of the implementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be animplementation-approximatedvalue representing theintegervalue that is represented by Z in radix-R notation.)
  15. If mathInt = 0, then
    1. If sign = -1, return-0𝔽.
    2. Return+0𝔽.
  16. Return𝔽(sign × mathInt).
Note

parseInt may interpret only a leading portion of string as anintegervalue; it ignores any code units that cannot be interpreted as part of the notation of aninteger, and no indication is given that any such code units were ignored.

19.2.6 URI Handling Functions

Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language itself does not provide any support for using URIs except for functions that encode and decode URIs as described in19.2.6.2,19.2.6.3,19.2.6.4and19.2.6.5

Note

Many implementations of ECMAScript provide additional functions and methods that manipulate web pages; these functions are beyond the scope of this standard.

19.2.6.1 URI Syntax and Semantics

A URI is composed of a sequence of components separated by component separators. The general form is:

Scheme : First / Second ; Third ? Fourth

where the italicized names represent components and “:”, “/”, “;” and “?” are reserved for use as separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they assume that any reserved code units in the URI are intended to have special meaning and so are not encoded. The encodeURIComponent and decodeURIComponent functions are intended to work with the individual component parts of a URI; they assume that any reserved code units represent text and so must be encoded so that they are not interpreted as reserved code units when the component is part of a complete URI.

The following lexical grammar specifies the form of encoded URIs.

Syntax

uri:::uriCharactersopturiCharacters:::uriCharacteruriCharactersopturiCharacter:::uriReserveduriUnescapeduriEscapeduriReserved:::one of;/?:@&=+$,uriUnescaped:::uriAlphaDecimalDigituriMarkuriEscaped:::%HexDigitHexDigituriAlpha:::one ofabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZuriMark:::one of-_.!~*'()Note

The above syntax is based upon RFC 2396 and does not reflect changes introduced by the more recent RFC 3986.

Runtime Semantics

When a code unit to be included in a URI is not listed above or is not intended to have the special meaning sometimes given to the reserved code units, that code unit must be encoded. The code unit is transformed into its UTF-8 encoding, withsurrogate pairsfirst converted from UTF-16 to the corresponding code point value. (Note that for code units in the range [0, 127] this results in a single octet with the same value.) The resulting sequence of octets is then transformed into a String with each octet represented by an escape sequence of the form"%xx".

19.2.6.1.1 Encode ( string, unescapedSet )

The abstract operation Encode takes arguments string (a String) and unescapedSet (a String). It performs URI encoding and escaping. It performs the following steps when called:

  1. Let strLen be the number of code units in string.
  2. Let R be the empty String.
  3. Let k be 0.
  4. Repeat,
    1. If k = strLen, return R.
    2. Let C be the code unit at index k within string.
    3. If C is in unescapedSet, then
      1. Set k to k + 1.
      2. Set R to thestring-concatenationof R and C.
    4. Else,
      1. Let cp be ! CodePointAt(string, k).
      2. If cp.[[IsUnpairedSurrogate]] istrue, throw aURIErrorexception.
      3. Set k to k + cp.[[CodeUnitCount]].
      4. Let Octets be theListof octets resulting by applying the UTF-8 transformation to cp.[[CodePoint]].
      5. For each element octet of Octets, do
        1. Set R to thestring-concatenationof:
          • R
          • "%"
          • the String representation of octet, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary

19.2.6.1.2 Decode ( string, reservedSet )

The abstract operation Decode takes arguments string (a String) and reservedSet (a String). It performs URI unescaping and decoding. It performs the following steps when called:

  1. Let strLen be the length of string.
  2. Let R be the empty String.
  3. Let k be 0.
  4. Repeat,
    1. If k = strLen, return R.
    2. Let C be the code unit at index k within string.
    3. If C is not the code unit 0x0025 (PERCENT SIGN), then
      1. Let S be the String value containing only the code unit C.
    4. Else,
      1. Let start be k.
      2. If k + 2 ≥ strLen, throw aURIErrorexception.
      3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw aURIErrorexception.
      4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
      5. Set k to k + 2.
      6. Let n be the number of leading 1 bits in B.
      7. If n = 0, then
        1. Let C be the code unit whose value is B.
        2. If C is not in reservedSet, then
          1. Let S be the String value containing only the code unit C.
        3. Else,
          1. Let S be thesubstringof string from start to k + 1.
      8. Else,
        1. If n = 1 or n > 4, throw aURIErrorexception.
        2. If k + (3 × (n - 1)) ≥ strLen, throw aURIErrorexception.
        3. Let Octets be aListwhose sole element is B.
        4. Let j be 1.
        5. Repeat, while j < n,
          1. Set k to k + 1.
          2. If the code unit at index k within string is not the code unit 0x0025 (PERCENT SIGN), throw aURIErrorexception.
          3. If the code units at index (k + 1) and (k + 2) within string do not represent hexadecimal digits, throw aURIErrorexception.
          4. Let B be the 8-bit value represented by the two hexadecimal digits at index (k + 1) and (k + 2).
          5. Set k to k + 2.
          6. Append B to Octets.
          7. Set j to j + 1.
        6. Assert: The length of Octets is n.
        7. If Octets does not contain a valid UTF-8 encoding of a Unicode code point, throw aURIErrorexception.
        8. Let V be the code point obtained by applying the UTF-8 transformation to Octets, that is, from aListof octets into a 21-bit value.
        9. Let S beUTF16EncodeCodePoint(V).
    5. Set R to thestring-concatenationof R and S.
    6. Set k to k + 1.
Note

This syntax of Uniform Resource Identifiers is based upon RFC 2396 and does not reflect the more recent RFC 3986 which replaces RFC 2396. A formal description and implementation of UTF-8 is given in RFC 3629.

In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a sequence of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n > 1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from the value of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript characters are specified inTable 53.

Table 53 (Informative): UTF-8 Encodings
Code Unit ValueRepresentation1st Octet2nd Octet3rd Octet4th Octet
0x0000 - 0x007F00000000 0zzzzzzz0zzzzzzz
0x0080 - 0x07FF00000yyy yyzzzzzz110yyyyy10zzzzzz
0x0800 - 0xD7FFxxxxyyyy yyzzzzzz1110xxxx10yyyyyy10zzzzzz
0xD800 - 0xDBFF
followed by
0xDC00 - 0xDFFF
110110vv vvwwwwxx
followed by
110111yy yyzzzzzz
11110uuu10uuwwww10xxyyyy10zzzzzz
0xD800 - 0xDBFF
not followed by
0xDC00 - 0xDFFF
causes URIError
0xDC00 - 0xDFFFcauses URIError
0xE000 - 0xFFFFxxxxyyyy yyzzzzzz1110xxxx10yyyyyy10zzzzzz

Where
uuuuu = vvvv + 1
to account for the addition of 0x10000 as in section 3.8 of the Unicode Standard (Surrogates).

The above transformation combines eachsurrogate pair(for which code unit values in the inclusive range 0xD800 to 0xDFFF are reserved) into a UTF-32 representation and encodes the resulting 21-bit value into UTF-8. Decoding reconstructs thesurrogate pair.

RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are required to throw aURIErrorwhen encountering such invalid sequences.

19.2.6.2 decodeURI ( encodedURI )

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding of the code points that it represents. Escape sequences that could not have been introduced by encodeURI are not replaced.

The decodeURI function is the %decodeURI% intrinsic object. When the decodeURI function is called with one argument encodedURI, the following steps are taken:

  1. Let uriString be ? ToString(encodedURI).
  2. Let reservedURISet be a String containing one instance of each code unit valid inuriReservedplus"#".
  3. Return ? Decode(uriString, reservedURISet).
Note

The code point # is not decoded from escape sequences even though it is not a reserved URI code point.

19.2.6.3 decodeURIComponent ( encodedURIComponent )

The decodeURIComponent function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16 encoding of the code points that it represents.

The decodeURIComponent function is the %decodeURIComponent% intrinsic object. When the decodeURIComponent function is called with one argument encodedURIComponent, the following steps are taken:

  1. Let componentString be ? ToString(encodedURIComponent).
  2. Let reservedURIComponentSet be the empty String.
  3. Return ? Decode(componentString, reservedURIComponentSet).

19.2.6.4 encodeURI ( uri )

The encodeURI function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code points.

The encodeURI function is the %encodeURI% intrinsic object. When the encodeURI function is called with one argument uri, the following steps are taken:

  1. Let uriString be ? ToString(uri).
  2. Let unescapedURISet be a String containing one instance of each code unit valid inuriReservedanduriUnescapedplus"#".
  3. Return ? Encode(uriString, unescapedURISet).
Note

The code point # is not encoded to an escape sequence even though it is not a reserved or unescaped URI code point.

19.2.6.5 encodeURIComponent ( uriComponent )

The encodeURIComponent function computes a new version of a UTF-16 encoded (6.1.4) URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences representing the UTF-8 encoding of the code point.

The encodeURIComponent function is the %encodeURIComponent% intrinsic object. When the encodeURIComponent function is called with one argument uriComponent, the following steps are taken:

  1. Let componentString be ? ToString(uriComponent).
  2. Let unescapedURIComponentSet be a String containing one instance of each code unit valid inuriUnescaped.
  3. Return ? Encode(componentString, unescapedURIComponentSet).

19.3 Constructor Properties of the Global Object

19.3.1 Array ( . . . )

See23.1.1.

19.3.2 ArrayBuffer ( . . . )

See25.1.3.

19.3.3 BigInt ( . . . )

See21.2.1.

19.3.4 BigInt64Array ( . . . )

See23.2.5.

19.3.5 BigUint64Array ( . . . )

See23.2.5.

19.3.6 Boolean ( . . . )

See20.3.1.

19.3.7 DataView ( . . . )

See25.3.2.

19.3.8 Date ( . . . )

See21.4.2.

19.3.9 Error ( . . . )

See20.5.1.

19.3.10 EvalError ( . . . )

See20.5.5.1.

19.3.11 FinalizationRegistry ( . . . )

See26.2.1.

19.3.12 Float32Array ( . . . )

See23.2.5.

19.3.13 Float64Array ( . . . )

See23.2.5.

19.3.14 Function ( . . . )

See20.2.1.

19.3.15 Int8Array ( . . . )

See23.2.5.

19.3.16 Int16Array ( . . . )

See23.2.5.

19.3.17 Int32Array ( . . . )

See23.2.5.

19.3.18 Map ( . . . )

See24.1.1.

19.3.19 Number ( . . . )

See21.1.1.

19.3.20 Object ( . . . )

See20.1.1.

19.3.21 Promise ( . . . )

See27.2.3.

19.3.22 Proxy ( . . . )

See28.2.1.

19.3.23 RangeError ( . . . )

See20.5.5.2.

19.3.24 ReferenceError ( . . . )

See20.5.5.3.

19.3.25 RegExp ( . . . )

See22.2.3.

19.3.26 Set ( . . . )

See24.2.1.

19.3.27 SharedArrayBuffer ( . . . )

See25.2.2.

19.3.28 String ( . . . )

See22.1.1.

19.3.29 Symbol ( . . . )

See20.4.1.

19.3.30 SyntaxError ( . . . )

See20.5.5.4.

19.3.31 TypeError ( . . . )

See20.5.5.5.

19.3.32 Uint8Array ( . . . )

See23.2.5.

19.3.33 Uint8ClampedArray ( . . . )

See23.2.5.

19.3.34 Uint16Array ( . . . )

See23.2.5.

19.3.35 Uint32Array ( . . . )

See23.2.5.

19.3.36 URIError ( . . . )

See20.5.5.6.

19.3.37 WeakMap ( . . . )

See24.3.1.

19.3.38 WeakRef ( . . . )

See26.1.1.

19.3.39 WeakSet ( . . . )

See24.4.

19.4 Other Properties of the Global Object

19.4.1 Atomics

See25.4.

19.4.2 JSON

See25.5.

19.4.3 Math

See21.3.

19.4.4 Reflect

See28.1.

20 Fundamental Objects

20.1 Object Objects

20.1.1 The Object Constructor

The Objectconstructor:

  • is %Object%.
  • is the initial value of the"Object"property of theglobal object.
  • creates a newordinary objectwhen called as aconstructor.
  • performs a type conversion when called as a function rather than as aconstructor.
  • may be used as the value of an extends clause of a class definition.

20.1.1.1 Object ( [ value ] )

When the Object function is called with optional argument value, the following steps are taken:

  1. If NewTarget is neitherundefinednor the active function, then
    1. Return ? OrdinaryCreateFromConstructor(NewTarget,"%Object.prototype%").
  2. If value isundefinedornull, return ! OrdinaryObjectCreate(%Object.prototype%).
  3. Return ! ToObject(value).

The"length"property of the Object function is1𝔽.

20.1.2 Properties of the Object Constructor

The Objectconstructor:

  • has a [[Prototype]] internal slot whose value is%Function.prototype%.
  • has a"length"property.
  • has the following additional properties:

20.1.2.1 Object.assign ( target, ...sources )

The assign function is used to copy the values of all of the enumerable own properties from one or more source objects to a target object. When the assign function is called, the following steps are taken:

  1. Let to be ? ToObject(target).
  2. If only one argument was passed, return to.
  3. For each element nextSource of sources, do
    1. If nextSource is neitherundefinednornull, then
      1. Let from be ! ToObject(nextSource).
      2. Let keys be ? from.[[OwnPropertyKeys]]().
      3. For each element nextKey of keys, do
        1. Let desc be ? from.[[GetOwnProperty]](nextKey).
        2. If desc is notundefinedand desc.[[Enumerable]] istrue, then
          1. Let propValue be ? Get(from, nextKey).
          2. Perform ? Set(to, nextKey, propValue,true).
  4. Return to.

The"length"property of the assign function is2𝔽.

20.1.2.2 Object.create ( O, Properties )

The create function creates a new object with a specified prototype. When the create function is called, the following steps are taken:

  1. IfType(O) is neither Object nor Null, throw aTypeErrorexception.
  2. Let obj be ! OrdinaryObjectCreate(O).
  3. If Properties is notundefined, then
    1. Return ? ObjectDefineProperties(obj, Properties).
  4. Return obj.

20.1.2.3 Object.defineProperties ( O, Properties )

The defineProperties function is used to add own properties and/or update the attributes of existing own properties of an object. When the defineProperties function is called, the following steps are taken:

  1. IfType(O) is not Object, throw aTypeErrorexception.
  2. Return ? ObjectDefineProperties(O, Properties).

20.1.2.3.1 ObjectDefineProperties ( O, Properties )

The abstract operation ObjectDefineProperties takes arguments O and Properties. It performs the following steps when called:

  1. Assert:Type(O) is Object.
  2. Let props be ? ToObject(Properties).
  3. Let keys be ? props.[[OwnPropertyKeys]]().
  4. Let descriptors be a new emptyList.
  5. For each element nextKey of keys, do
    1. Let propDesc be ? props.[[GetOwnProperty]](nextKey).
    2. If propDesc is notundefinedand propDesc.[[Enumerable]] istrue, then
      1. Let descObj be ? Get(props, nextKey).
      2. Let desc be ? ToPropertyDescriptor(descObj).
      3. Append the pair (a two elementList) consisting of nextKey and desc to the end of descriptors.
  6. For each element pair of descriptors, do
    1. Let P be the first element of pair.
    2. Let desc be the second element of pair.
    3. Perform ? DefinePropertyOrThrow(O, P, desc).
  7. Return O.

20.1.2.4 Object.defineProperty ( O, P, Attributes )

The defineProperty function is used to add an own property and/or update the attributes of an existing own property of an object. When the defineProperty function is called, the following steps are taken:

  1. IfType(O) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? ToPropertyDescriptor(Attributes).
  4. Perform ? DefinePropertyOrThrow(O, key, desc).
  5. Return O.

20.1.2.5 Object.entries ( O )

When the entries function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Let nameList be ? EnumerableOwnPropertyNames(obj,key+value).
  3. ReturnCreateArrayFromList(nameList).

20.1.2.6 Object.freeze ( O )

When the freeze function is called, the following steps are taken:

  1. IfType(O) is not Object, return O.
  2. Let status be ? SetIntegrityLevel(O,frozen).
  3. If status isfalse, throw aTypeErrorexception.
  4. Return O.

20.1.2.7 Object.fromEntries ( iterable )

When the fromEntries method is called with argument iterable, the following steps are taken:

  1. Perform ? RequireObjectCoercible(iterable).
  2. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  3. Assert: obj is an extensibleordinary objectwith no own properties.
  4. Let closure be a newAbstract Closurewith parameters (key, value) that captures obj and performs the following steps when called:
    1. Let propertyKey be ? ToPropertyKey(key).
    2. Perform ! CreateDataPropertyOrThrow(obj, propertyKey, value).
    3. Returnundefined.
  5. Let adder be ! CreateBuiltinFunction(closure, 2,"", « »).
  6. Return ? AddEntriesFromIterable(obj, iterable, adder).
Note
The function created for adder is never directly accessible to ECMAScript code.

20.1.2.8 Object.getOwnPropertyDescriptor ( O, P )

When the getOwnPropertyDescriptor function is called, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Let key be ? ToPropertyKey(P).
  3. Let desc be ? obj.[[GetOwnProperty]](key).
  4. ReturnFromPropertyDescriptor(desc).

20.1.2.9 Object.getOwnPropertyDescriptors ( O )

When the getOwnPropertyDescriptors function is called, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Let ownKeys be ? obj.[[OwnPropertyKeys]]().
  3. Let descriptors be ! OrdinaryObjectCreate(%Object.prototype%).
  4. For each element key of ownKeys, do
    1. Let desc be ? obj.[[GetOwnProperty]](key).
    2. Let descriptor be ! FromPropertyDescriptor(desc).
    3. If descriptor is notundefined, perform ! CreateDataPropertyOrThrow(descriptors, key, descriptor).
  5. Return descriptors.

20.1.2.10 Object.getOwnPropertyNames ( O )

When the getOwnPropertyNames function is called, the following steps are taken:

  1. Return ? GetOwnPropertyKeys(O,string).

20.1.2.11 Object.getOwnPropertySymbols ( O )

When the getOwnPropertySymbols function is called with argument O, the following steps are taken:

  1. Return ? GetOwnPropertyKeys(O,symbol).

20.1.2.11.1 GetOwnPropertyKeys ( O, type )

The abstract operation GetOwnPropertyKeys takes arguments O and type (eitherstringorsymbol). It performs the following steps when called:

  1. Let obj be ? ToObject(O).
  2. Let keys be ? obj.[[OwnPropertyKeys]]().
  3. Let nameList be a new emptyList.
  4. For each element nextKey of keys, do
    1. IfType(nextKey) is Symbol and type issymbolorType(nextKey) is String and type isstring, then
      1. Append nextKey as the last element of nameList.
  5. ReturnCreateArrayFromList(nameList).

20.1.2.12 Object.getPrototypeOf ( O )

When the getPrototypeOf function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Return ? obj.[[GetPrototypeOf]]().

20.1.2.13 Object.is ( value1, value2 )

When the is function is called with arguments value1 and value2, the following steps are taken:

  1. ReturnSameValue(value1, value2).

20.1.2.14 Object.isExtensible ( O )

When the isExtensible function is called with argument O, the following steps are taken:

  1. IfType(O) is not Object, returnfalse.
  2. Return ? IsExtensible(O).

20.1.2.15 Object.isFrozen ( O )

When the isFrozen function is called with argument O, the following steps are taken:

  1. IfType(O) is not Object, returntrue.
  2. Return ? TestIntegrityLevel(O,frozen).

20.1.2.16 Object.isSealed ( O )

When the isSealed function is called with argument O, the following steps are taken:

  1. IfType(O) is not Object, returntrue.
  2. Return ? TestIntegrityLevel(O,sealed).

20.1.2.17 Object.keys ( O )

When the keys function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Let nameList be ? EnumerableOwnPropertyNames(obj,key).
  3. ReturnCreateArrayFromList(nameList).

20.1.2.18 Object.preventExtensions ( O )

When the preventExtensions function is called, the following steps are taken:

  1. IfType(O) is not Object, return O.
  2. Let status be ? O.[[PreventExtensions]]().
  3. If status isfalse, throw aTypeErrorexception.
  4. Return O.

20.1.2.19 Object.prototype

The initial value of Object.prototype is theObject prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.1.2.20 Object.seal ( O )

When the seal function is called, the following steps are taken:

  1. IfType(O) is not Object, return O.
  2. Let status be ? SetIntegrityLevel(O,sealed).
  3. If status isfalse, throw aTypeErrorexception.
  4. Return O.

20.1.2.21 Object.setPrototypeOf ( O, proto )

When the setPrototypeOf function is called with arguments O and proto, the following steps are taken:

  1. Set O to ? RequireObjectCoercible(O).
  2. IfType(proto) is neither Object nor Null, throw aTypeErrorexception.
  3. IfType(O) is not Object, return O.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status isfalse, throw aTypeErrorexception.
  6. Return O.

20.1.2.22 Object.values ( O )

When the values function is called with argument O, the following steps are taken:

  1. Let obj be ? ToObject(O).
  2. Let nameList be ? EnumerableOwnPropertyNames(obj,value).
  3. ReturnCreateArrayFromList(nameList).

20.1.3 Properties of the Object Prototype Object

The Object prototype object:

  • is %Object.prototype%.
  • has an [[Extensible]] internal slot whose value istrue.
  • has the internal methods defined for ordinary objects, except for the [[SetPrototypeOf]] method, which is as defined in10.4.7.1. (Thus, it is animmutable prototype exotic object.)
  • has a [[Prototype]] internal slot whose value isnull.

20.1.3.1 Object.prototype.constructor

The initial value of Object.prototype.constructor is%Object%.

20.1.3.2 Object.prototype.hasOwnProperty ( V )

When the hasOwnProperty method is called with argument V, the following steps are taken:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(thisvalue).
  3. Return ? HasOwnProperty(O, P).
Note

The ordering of steps1and2is chosen to ensure that any exception that would have been thrown by step1in previous editions of this specification will continue to be thrown even if thethisvalue isundefinedornull.

20.1.3.3 Object.prototype.isPrototypeOf ( V )

When the isPrototypeOf method is called with argument V, the following steps are taken:

  1. IfType(V) is not Object, returnfalse.
  2. Let O be ? ToObject(thisvalue).
  3. Repeat,
    1. Set V to ? V.[[GetPrototypeOf]]().
    2. If V isnull, returnfalse.
    3. IfSameValue(O, V) istrue, returntrue.
Note

The ordering of steps1and2preserves the behaviour specified by previous editions of this specification for the case where V is not an object and thethisvalue isundefinedornull.

20.1.3.4 Object.prototype.propertyIsEnumerable ( V )

When the propertyIsEnumerable method is called with argument V, the following steps are taken:

  1. Let P be ? ToPropertyKey(V).
  2. Let O be ? ToObject(thisvalue).
  3. Let desc be ? O.[[GetOwnProperty]](P).
  4. If desc isundefined, returnfalse.
  5. Return desc.[[Enumerable]].
Note 1

This method does not consider objects in the prototype chain.

Note 2

The ordering of steps1and2is chosen to ensure that any exception that would have been thrown by step1in previous editions of this specification will continue to be thrown even if thethisvalue isundefinedornull.

20.1.3.5 Object.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

When the toLocaleString method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Return ? Invoke(O,"toString").

The optional parameters to this function are not used but are intended to correspond to the parameter pattern used by ECMA-402 toLocaleString functions. Implementations that do not include ECMA-402 support must not use those parameter positions for other purposes.

Note 1

This function provides a generic toLocaleString implementation for objects that have no locale-specific toString behaviour. Array, Number, Date, and%TypedArray%provide their own locale-sensitive toLocaleString methods.

Note 2

ECMA-402 intentionally does not provide an alternative to this default implementation.

20.1.3.6 Object.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. If thethisvalue isundefined, return"[object Undefined]".
  2. If thethisvalue isnull, return"[object Null]".
  3. Let O be ! ToObject(thisvalue).
  4. Let isArray be ? IsArray(O).
  5. If isArray istrue, let builtinTag be"Array".
  6. Else if O has a [[ParameterMap]] internal slot, let builtinTag be"Arguments".
  7. Else if O has a [[Call]] internal method, let builtinTag be"Function".
  8. Else if O has an [[ErrorData]] internal slot, let builtinTag be"Error".
  9. Else if O has a [[BooleanData]] internal slot, let builtinTag be"Boolean".
  10. Else if O has a [[NumberData]] internal slot, let builtinTag be"Number".
  11. Else if O has a [[StringData]] internal slot, let builtinTag be"String".
  12. Else if O has a [[DateValue]] internal slot, let builtinTag be"Date".
  13. Else if O has a [[RegExpMatcher]] internal slot, let builtinTag be"RegExp".
  14. Else, let builtinTag be"Object".
  15. Let tag be ? Get(O,@@toStringTag).
  16. IfType(tag) is not String, set tag to builtinTag.
  17. Return thestring-concatenationof"[object ", tag, and"]".
Note

Historically, this function was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification as a nominal type tag for various built-in objects. The above definition of toString preserves compatibility for legacy code that uses toString as a test for those specific kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of built-in or program defined objects. In addition, programs can use@@toStringTagin ways that will invalidate the reliability of such legacy type tests.

20.1.3.7 Object.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

  1. Return ? ToObject(thisvalue).

20.1.3.8 Object.prototype.__proto__

Object.prototype.__proto__ is anaccessor propertywith attributes { [[Enumerable]]:false, [[Configurable]]:true}. The [[Get]] and [[Set]] attributes are defined as follows:

20.1.3.8.1 get Object.prototype.__proto__

The value of the [[Get]] attribute is a built-in function that requires no arguments. It performs the following steps when called:

  1. Let O be ? ToObject(thisvalue).
  2. Return ? O.[[GetPrototypeOf]]().

20.1.3.8.2 set Object.prototype.__proto__

The value of the [[Set]] attribute is a built-in function that takes an argument proto. It performs the following steps when called:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. IfType(proto) is neither Object nor Null, returnundefined.
  3. IfType(O) is not Object, returnundefined.
  4. Let status be ? O.[[SetPrototypeOf]](proto).
  5. If status isfalse, throw aTypeErrorexception.
  6. Returnundefined.

20.1.3.9 Legacy Object.prototype Accessor Methods

20.1.3.9.1 Object.prototype.__defineGetter__ ( P, getter )

When the __defineGetter__ method is called with arguments P and getter, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. IfIsCallable(getter) isfalse, throw aTypeErrorexception.
  3. Let desc be PropertyDescriptor { [[Get]]: getter, [[Enumerable]]:true, [[Configurable]]:true}.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Returnundefined.

20.1.3.9.2 Object.prototype.__defineSetter__ ( P, setter )

When the __defineSetter__ method is called with arguments P and setter, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. IfIsCallable(setter) isfalse, throw aTypeErrorexception.
  3. Let desc be PropertyDescriptor { [[Set]]: setter, [[Enumerable]]:true, [[Configurable]]:true}.
  4. Let key be ? ToPropertyKey(P).
  5. Perform ? DefinePropertyOrThrow(O, key, desc).
  6. Returnundefined.

20.1.3.9.3 Object.prototype.__lookupGetter__ ( P )

When the __lookupGetter__ method is called with argument P, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is notundefined, then
      1. IfIsAccessorDescriptor(desc) istrue, return desc.[[Get]].
      2. Returnundefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O isnull, returnundefined.

20.1.3.9.4 Object.prototype.__lookupSetter__ ( P )

When the __lookupSetter__ method is called with argument P, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let key be ? ToPropertyKey(P).
  3. Repeat,
    1. Let desc be ? O.[[GetOwnProperty]](key).
    2. If desc is notundefined, then
      1. IfIsAccessorDescriptor(desc) istrue, return desc.[[Set]].
      2. Returnundefined.
    3. Set O to ? O.[[GetPrototypeOf]]().
    4. If O isnull, returnundefined.

20.1.4 Properties of Object Instances

Object instances have no special properties beyond those inherited from theObject prototype object.

20.2 Function Objects

20.2.1 The Function Constructor

The Functionconstructor:

  • is %Function%.
  • is the initial value of the"Function"property of theglobal object.
  • creates and initializes a newfunction objectwhen called as a function rather than as aconstructor. Thus the function call Function(…) is equivalent to the object creation expression new Function(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Function behaviour must include a super call to the Functionconstructorto create and initialize a subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript syntactic forms for defining function objects create instances of Function. There is no syntactic means to create instances of Function subclasses except for the built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.

20.2.1.1 Function ( p1, p2, … , pn, body )

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal parameters.

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “ p ” arguments, and where body might also not be provided), the following steps are taken:

  1. Let C be theactive function object.
  2. Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
  3. Return ? CreateDynamicFunction(C, NewTarget,normal, args).
Note

It is permissible but not necessary to have one argument for each formal parameter to be specified. For example, all three of the following expressions produce the same result:

new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

20.2.1.1.1 CreateDynamicFunction ( constructor, newTarget, kind, args )

The abstract operation CreateDynamicFunction takes arguments constructor (aconstructor), newTarget (aconstructor), kind (eithernormal,generator,async, orasyncGenerator), and args (aListof ECMAScript language values). constructor is theconstructorfunction that is performing this action. newTarget is theconstructorthat new was initially applied to. args is the argument values that were passed to constructor. It performs the following steps when called:

  1. Assert: Theexecution context stackhas at least two elements.
  2. Let callerContext be the second to top element of theexecution context stack.
  3. Let callerRealm be callerContext'sRealm.
  4. Let calleeRealm bethe current Realm Record.
  5. Perform ? HostEnsureCanCompileStrings(callerRealm, calleeRealm).
  6. If newTarget isundefined, set newTarget to constructor.
  7. If kind isnormal, then
    1. Let prefix be"function".
    2. Let exprSym be the grammar symbolFunctionExpression.
    3. Let bodySym be the grammar symbolFunctionBody[~Yield, ~Await].
    4. Let parameterSym be the grammar symbolFormalParameters[~Yield, ~Await].
    5. Let fallbackProto be"%Function.prototype%".
  8. Else if kind isgenerator, then
    1. Let prefix be"function*".
    2. Let exprSym be the grammar symbolGeneratorExpression.
    3. Let bodySym be the grammar symbolGeneratorBody.
    4. Let parameterSym be the grammar symbolFormalParameters[+Yield, ~Await].
    5. Let fallbackProto be"%GeneratorFunction.prototype%".
  9. Else if kind isasync, then
    1. Let prefix be"async function".
    2. Let exprSym be the grammar symbolAsyncFunctionExpression.
    3. Let bodySym be the grammar symbolAsyncFunctionBody.
    4. Let parameterSym be the grammar symbolFormalParameters[~Yield, +Await].
    5. Let fallbackProto be"%AsyncFunction.prototype%".
  10. Else,
    1. Assert: kind isasyncGenerator.
    2. Let prefix be"async function*".
    3. Let exprSym be the grammar symbolAsyncGeneratorExpression.
    4. Let bodySym be the grammar symbolAsyncGeneratorBody.
    5. Let parameterSym be the grammar symbolFormalParameters[+Yield, +Await].
    6. Let fallbackProto be"%AsyncGeneratorFunction.prototype%".
  11. Let argCount be the number of elements in args.
  12. Let P be the empty String.
  13. If argCount = 0, let bodyArg be the empty String.
  14. Else if argCount = 1, let bodyArg be args[0].
  15. Else,
    1. Assert: argCount > 1.
    2. Let firstArg be args[0].
    3. Set P to ? ToString(firstArg).
    4. Let k be 1.
    5. Repeat, while k < argCount - 1,
      1. Let nextArg be args[k].
      2. Let nextArgString be ? ToString(nextArg).
      3. Set P to thestring-concatenationof P,","(a comma), and nextArgString.
      4. Set k to k + 1.
    6. Let bodyArg be args[k].
  16. Let bodyString be thestring-concatenationof 0x000A (LINE FEED), ? ToString(bodyArg), and 0x000A (LINE FEED).
  17. Let sourceString be thestring-concatenationof prefix," anonymous(", P, 0x000A (LINE FEED),") {", bodyString, and"}".
  18. Let sourceText be ! StringToCodePoints(sourceString).
  19. Let parameters beParseText(!StringToCodePoints(P), parameterSym).
  20. If parameters is aListof errors, throw aSyntaxErrorexception.
  21. Let body beParseText(!StringToCodePoints(bodyString), bodySym).
  22. If body is aListof errors, throw aSyntaxErrorexception.
  23. NOTE: The parameters and body are parsed separately to ensure that each is valid alone. For example, new Function("/*", "*/ ) {") is not legal.
  24. NOTE: If this step is reached, sourceText must match exprSym (although the reverse implication does not hold). The purpose of the next two steps is to enforce any Early Error rules which apply to exprSym directly.
  25. Let expr beParseText(sourceText, exprSym).
  26. If expr is aListof errors, throw aSyntaxErrorexception.
  27. Let proto be ? GetPrototypeFromConstructor(newTarget, fallbackProto).
  28. Let realmF bethe current Realm Record.
  29. Let scope be realmF.[[GlobalEnv]].
  30. Let privateScope benull.
  31. Let F be ! OrdinaryFunctionCreate(proto, sourceText, parameters, body,non-lexical-this, scope, privateScope).
  32. PerformSetFunctionName(F,"anonymous").
  33. If kind isgenerator, then
    1. Let prototype be ! OrdinaryObjectCreate(%GeneratorFunction.prototype.prototype%).
    2. PerformDefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  34. Else if kind isasyncGenerator, then
    1. Let prototype be ! OrdinaryObjectCreate(%AsyncGeneratorFunction.prototype.prototype%).
    2. PerformDefinePropertyOrThrow(F,"prototype", PropertyDescriptor { [[Value]]: prototype, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  35. Else if kind isnormal, performMakeConstructor(F).
  36. NOTE: Functions whose kind isasyncare not constructible and do not have a [[Construct]] internal method or a"prototype"property.
  37. Return F.
Note

CreateDynamicFunction defines a"prototype"property on any function it creates whose kind is notasyncto provide for the possibility that the function will be used as aconstructor.

20.2.2 Properties of the Function Constructor

The Functionconstructor:

20.2.2.1 Function.length

This is adata propertywith a value of 1. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

20.2.2.2 Function.prototype

The value of Function.prototype is theFunction prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.2.3 Properties of the Function Prototype Object

The Function prototype object:

  • is %Function.prototype%.
  • is itself a built-infunction object.
  • accepts any arguments and returnsundefinedwhen invoked.
  • does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • does not have a"prototype"property.
  • has a"length"property whose value is+0𝔽.
  • has a"name"property whose value is the empty String.
Note

The Function prototype object is specified to be afunction objectto ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

20.2.3.1 Function.prototype.apply ( thisArg, argArray )

When the apply method is called with arguments thisArg and argArray, the following steps are taken:

  1. Let func be thethisvalue.
  2. IfIsCallable(func) isfalse, throw aTypeErrorexception.
  3. If argArray isundefinedornull, then
    1. PerformPrepareForTailCall().
    2. Return ? Call(func, thisArg).
  4. Let argList be ? CreateListFromArrayLike(argArray).
  5. PerformPrepareForTailCall().
  6. Return ? Call(func, thisArg, argList).
Note 1

The thisArg value is passed without modification as thethisvalue. This is a change from Edition 3, where anundefinedornullthisArg is replaced with theglobal objectandToObjectis applied to all other values and that result is passed as thethisvalue. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

Note 2

If func is an arrow function or abound function exotic objectthen the thisArg will be ignored by the function [[Call]] in step6.

20.2.3.2 Function.prototype.bind ( thisArg, ...args )

When the bind method is called with argument thisArg and zero or more args, it performs the following steps:

  1. Let Target be thethisvalue.
  2. IfIsCallable(Target) isfalse, throw aTypeErrorexception.
  3. Let F be ? BoundFunctionCreate(Target, thisArg, args).
  4. Let L be 0.
  5. Let targetHasLength be ? HasOwnProperty(Target,"length").
  6. If targetHasLength istrue, then
    1. Let targetLen be ? Get(Target,"length").
    2. IfType(targetLen) is Number, then
      1. If targetLen is+∞𝔽, set L to +∞.
      2. Else if targetLen is-∞𝔽, set L to 0.
      3. Else,
        1. Let targetLenAsInt be ! ToIntegerOrInfinity(targetLen).
        2. Assert: targetLenAsInt is finite.
        3. Let argCount be the number of elements in args.
        4. Set L tomax(targetLenAsInt - argCount, 0).
  7. Perform ! SetFunctionLength(F, L).
  8. Let targetName be ? Get(Target,"name").
  9. IfType(targetName) is not String, set targetName to the empty String.
  10. PerformSetFunctionName(F, targetName,"bound").
  11. Return F.
Note 1

Function objects created using Function.prototype.bind are exotic objects. They also do not have a"prototype"property.

Note 2

If Target is an arrow function or abound function exotic objectthen the thisArg passed to this method will not be used by subsequent calls to F.

20.2.3.3 Function.prototype.call ( thisArg, ...args )

When the call method is called with argument thisArg and zero or more args, the following steps are taken:

  1. Let func be thethisvalue.
  2. IfIsCallable(func) isfalse, throw aTypeErrorexception.
  3. PerformPrepareForTailCall().
  4. Return ? Call(func, thisArg, args).
Note 1

The thisArg value is passed without modification as thethisvalue. This is a change from Edition 3, where anundefinedornullthisArg is replaced with theglobal objectandToObjectis applied to all other values and that result is passed as thethisvalue. Even though the thisArg is passed without modification, non-strict functions still perform these transformations upon entry to the function.

Note 2

If func is an arrow function or abound function exotic objectthen the thisArg will be ignored by the function [[Call]] in step4.

20.2.3.4 Function.prototype.constructor

The initial value of Function.prototype.constructor is%Function%.

20.2.3.5 Function.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. Let func be thethisvalue.
  2. IfType(func) is Object and func has a [[SourceText]] internal slot and func.[[SourceText]] is a sequence of Unicode code points and ! HostHasSourceTextAvailable(func) istrue, then
    1. Return ! CodePointsToString(func.[[SourceText]]).
  3. If func is abuilt-in function object, return animplementation-definedString source code representation of func. The representation must have the syntax of aNativeFunction. Additionally, if func has an [[InitialName]] internal slot and func.[[InitialName]] is a String, the portion of the returned String that would be matched byNativeFunctionAccessoroptPropertyNamemust be the value of func.[[InitialName]].
  4. IfType(func) is Object andIsCallable(func) istrue, return animplementation-definedString source code representation of func. The representation must have the syntax of aNativeFunction.
  5. Throw aTypeErrorexception.
NativeFunction:functionNativeFunctionAccessoroptPropertyName[~Yield, ~Await]opt(FormalParameters[~Yield, ~Await]){[nativecode]}NativeFunctionAccessor:getset

20.2.3.6 Function.prototype [ @@hasInstance ] ( V )

When the @@hasInstance method of an object F is called with value V, the following steps are taken:

  1. Let F be thethisvalue.
  2. Return ? OrdinaryHasInstance(F, V).

The value of the"name"property of this function is"[Symbol.hasInstance]".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

Note

This is the default implementation of @@hasInstance that most functions inherit. @@hasInstance is called by the instanceof operator to determine whether a value is an instance of a specificconstructor. An expression such as

v instanceof F

evaluates as

F[@@hasInstance](v)

Aconstructorfunction can control which objects are recognized as its instances by instanceof by exposing a different @@hasInstance method on the function.

This property is non-writable and non-configurable to prevent tampering that could be used to globally expose the target function of a bound function.

20.2.4 Function Instances

Every Function instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. Function objects created using the Function.prototype.bind method (20.2.3.2) have the internal slots listed inTable 34.

Function instances have the following properties:

20.2.4.1 length

The value of the"length"property is anintegral Numberthat indicates the typical number of arguments expected by the function. However, the language permits the function to be invoked with some other number of arguments. The behaviour of a function when invoked on a number of arguments other than the number specified by its"length"property depends on the function. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

20.2.4.2 name

The value of the"name"property is a String that is descriptive of the function. The name has no semantic significance but is typically a variable orproperty namethat is used to refer to the function at its point of definition in ECMAScript code. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

Anonymous functions objects that do not have a contextual name associated with them by this specification use the empty String as the value of the"name"property.

20.2.4.3 prototype

Function instances that can be used as aconstructorhave a"prototype"property. Whenever such a Function instance is created anotherordinary objectis also created and is the initial value of the function's"prototype"property. Unless otherwise specified, the value of the"prototype"property is used to initialize the [[Prototype]] internal slot of the object created when that function is invoked as aconstructor.

This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.

Note

Function objects created using Function.prototype.bind, or by evaluating aMethodDefinition(that is not aGeneratorMethodorAsyncGeneratorMethod) or anArrowFunctiondo not have a"prototype"property.

20.2.5 HostHasSourceTextAvailable ( func )

Thehost-definedabstract operation HostHasSourceTextAvailable takes argument func (afunction object). It allowshostenvironments to prevent the source text from being provided for func.

An implementation of HostHasSourceTextAvailable must conform to the following requirements:

  • It must complete normally (i.e. not return anabrupt completion).
  • It must be deterministic with respect to its parameters. Each time it is called with a specific func as its argument, it must return the same completion record.

The default implementation of HostHasSourceTextAvailable is to returnNormalCompletion(true).

20.3 Boolean Objects

20.3.1 The Boolean Constructor

The Booleanconstructor:

  • is %Boolean%.
  • is the initial value of the"Boolean"property of theglobal object.
  • creates and initializes a new Boolean object when called as aconstructor.
  • performs a type conversion when called as a function rather than as aconstructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Boolean behaviour must include a super call to the Booleanconstructorto create and initialize the subclass instance with a [[BooleanData]] internal slot.

20.3.1.1 Boolean ( value )

When Boolean is called with argument value, the following steps are taken:

  1. Let b be ! ToBoolean(value).
  2. If NewTarget isundefined, return b.
  3. Let O be ? OrdinaryCreateFromConstructor(NewTarget,"%Boolean.prototype%", « [[BooleanData]] »).
  4. Set O.[[BooleanData]] to b.
  5. Return O.

20.3.2 Properties of the Boolean Constructor

The Booleanconstructor:

20.3.2.1 Boolean.prototype

The initial value of Boolean.prototype is theBoolean prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.3.3 Properties of the Boolean Prototype Object

The Boolean prototype object:

  • is %Boolean.prototype%.
  • is anordinary object.
  • is itself a Boolean object; it has a [[BooleanData]] internal slot with the valuefalse.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

The abstract operation thisBooleanValue takes argument value. It performs the following steps when called:

  1. IfType(value) is Boolean, return value.
  2. IfType(value) is Object and value has a [[BooleanData]] internal slot, then
    1. Let b be value.[[BooleanData]].
    2. Assert:Type(b) is Boolean.
    3. Return b.
  3. Throw aTypeErrorexception.

20.3.3.1 Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is%Boolean%.

20.3.3.2 Boolean.prototype.toString ( )

The following steps are taken:

  1. Let b be ? thisBooleanValue(thisvalue).
  2. If b istrue, return"true"; else return"false".

20.3.3.3 Boolean.prototype.valueOf ( )

The following steps are taken:

  1. Return ? thisBooleanValue(thisvalue).

20.3.4 Properties of Boolean Instances

Boolean instances are ordinary objects that inherit properties from theBoolean prototype object. Boolean instances have a [[BooleanData]] internal slot. The [[BooleanData]] internal slot is the Boolean value represented by this Boolean object.

20.4 Symbol Objects

20.4.1 The Symbol Constructor

The Symbolconstructor:

  • is %Symbol%.
  • is the initial value of the"Symbol"property of theglobal object.
  • returns a new Symbol value when called as a function.
  • is not intended to be used with the new operator.
  • is not intended to be subclassed.
  • may be used as the value of an extends clause of a class definition but a super call to it will cause an exception.

20.4.1.1 Symbol ( [ description ] )

When Symbol is called with optional argument description, the following steps are taken:

  1. If NewTarget is notundefined, throw aTypeErrorexception.
  2. If description isundefined, let descString beundefined.
  3. Else, let descString be ? ToString(description).
  4. Return a new unique Symbol value whose [[Description]] value is descString.

20.4.2 Properties of the Symbol Constructor

The Symbolconstructor:

20.4.2.1 Symbol.asyncIterator

The initial value of Symbol.asyncIterator is the well known symbol@@asyncIterator(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.2 Symbol.for ( key )

When Symbol.for is called with argument key it performs the following steps:

  1. Let stringKey be ? ToString(key).
  2. For each element e of the GlobalSymbolRegistryList, do
    1. IfSameValue(e.[[Key]], stringKey) istrue, return e.[[Symbol]].
  3. Assert: GlobalSymbolRegistry does not currently contain an entry for stringKey.
  4. Let newSymbol be a new unique Symbol value whose [[Description]] value is stringKey.
  5. Append theRecord{ [[Key]]: stringKey, [[Symbol]]: newSymbol } to the GlobalSymbolRegistryList.
  6. Return newSymbol.

The GlobalSymbolRegistry is aListthat is globally available. It is shared by all realms. Prior to the evaluation of any ECMAScript code it is initialized as a new emptyList. Elements of the GlobalSymbolRegistry are Records with the structure defined inTable 54.

Table 54: GlobalSymbolRegistryRecordFields
Field NameValueUsage
[[Key]]A StringA string key used to globally identify a Symbol.
[[Symbol]]A SymbolA symbol that can be retrieved from anyrealm.

20.4.2.3 Symbol.hasInstance

The initial value of Symbol.hasInstance is the well-known symbol@@hasInstance(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.4 Symbol.isConcatSpreadable

The initial value of Symbol.isConcatSpreadable is the well-known symbol@@isConcatSpreadable(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.5 Symbol.iterator

The initial value of Symbol.iterator is the well-known symbol@@iterator(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.6 Symbol.keyFor ( sym )

When Symbol.keyFor is called with argument sym it performs the following steps:

  1. IfType(sym) is not Symbol, throw aTypeErrorexception.
  2. For each element e of the GlobalSymbolRegistryList(see20.4.2.2), do
    1. IfSameValue(e.[[Symbol]], sym) istrue, return e.[[Key]].
  3. Assert: GlobalSymbolRegistry does not currently contain an entry for sym.
  4. Returnundefined.

20.4.2.7 Symbol.match

The initial value of Symbol.match is the well-known symbol@@match(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.8 Symbol.matchAll

The initial value of Symbol.matchAll is the well-known symbol@@matchAll(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.9 Symbol.prototype

The initial value of Symbol.prototype is theSymbol prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.10 Symbol.replace

The initial value of Symbol.replace is the well-known symbol@@replace(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.11 Symbol.search

The initial value of Symbol.search is the well-known symbol@@search(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.12 Symbol.species

The initial value of Symbol.species is the well-known symbol@@species(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.13 Symbol.split

The initial value of Symbol.split is the well-known symbol@@split(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.14 Symbol.toPrimitive

The initial value of Symbol.toPrimitive is the well-known symbol@@toPrimitive(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.15 Symbol.toStringTag

The initial value of Symbol.toStringTag is the well-known symbol@@toStringTag(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.2.16 Symbol.unscopables

The initial value of Symbol.unscopables is the well-known symbol@@unscopables(Table 1).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.4.3 Properties of the Symbol Prototype Object

The Symbol prototype object:

  • is %Symbol.prototype%.
  • is anordinary object.
  • is not a Symbol instance and does not have a [[SymbolData]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

The abstract operation thisSymbolValue takes argument value. It performs the following steps when called:

  1. IfType(value) is Symbol, return value.
  2. IfType(value) is Object and value has a [[SymbolData]] internal slot, then
    1. Let s be value.[[SymbolData]].
    2. Assert:Type(s) is Symbol.
    3. Return s.
  3. Throw aTypeErrorexception.

20.4.3.1 Symbol.prototype.constructor

The initial value of Symbol.prototype.constructor is%Symbol%.

20.4.3.2 get Symbol.prototype.description

Symbol.prototype.description is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let s be thethisvalue.
  2. Let sym be ? thisSymbolValue(s).
  3. Return sym.[[Description]].

20.4.3.3 Symbol.prototype.toString ( )

The following steps are taken:

  1. Let sym be ? thisSymbolValue(thisvalue).
  2. ReturnSymbolDescriptiveString(sym).

20.4.3.3.1 SymbolDescriptiveString ( sym )

The abstract operation SymbolDescriptiveString takes argument sym. It performs the following steps when called:

  1. Assert:Type(sym) is Symbol.
  2. Let desc be sym's [[Description]] value.
  3. If desc isundefined, set desc to the empty String.
  4. Assert:Type(desc) is String.
  5. Return thestring-concatenationof"Symbol(", desc, and")".

20.4.3.4 Symbol.prototype.valueOf ( )

The following steps are taken:

  1. Return ? thisSymbolValue(thisvalue).

20.4.3.5 Symbol.prototype [ @@toPrimitive ] ( hint )

This function is called by ECMAScript language operators to convert a Symbol object to a primitive value.

When the @@toPrimitive method is called with argument hint, the following steps are taken:

  1. Return ? thisSymbolValue(thisvalue).

The value of the"name"property of this function is"[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

Note

The argument is ignored.

20.4.3.6 Symbol.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Symbol".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

20.4.4 Properties of Symbol Instances

Symbol instances are ordinary objects that inherit properties from theSymbol prototype object. Symbol instances have a [[SymbolData]] internal slot. The [[SymbolData]] internal slot is the Symbol value represented by this Symbol object.

20.5 Error Objects

Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also serve as base objects for user-defined exception classes.

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in20.5.5or a new instance of AggregateError object defined in20.5.7. Each of these objects has the structure described below, differing only in the name used as theconstructorname instead of NativeError, in the name property of the prototype object, in theimplementation-definedmessage property of the prototype object, and in the presence of the%AggregateError%-specific errors property.

20.5.1 The Error Constructor

The Errorconstructor:

  • is %Error%.
  • is the initial value of the"Error"property of theglobal object.
  • creates and initializes a new Error object when called as a function rather than as aconstructor. Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Errorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.1.1 Error ( message )

When the Error function is called with argument message, the following steps are taken:

  1. If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget,"%Error.prototype%", « [[ErrorData]] »).
  3. If message is notundefined, then
    1. Let msg be ? ToString(message).
    2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
    3. Perform ! DefinePropertyOrThrow(O,"message", msgDesc).
  4. Return O.

20.5.2 Properties of the Error Constructor

The Errorconstructor:

20.5.2.1 Error.prototype

The initial value of Error.prototype is theError prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.5.3 Properties of the Error Prototype Object

The Error prototype object:

  • is %Error.prototype%.
  • is anordinary object.
  • is not an Error instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

20.5.3.1 Error.prototype.constructor

The initial value of Error.prototype.constructor is%Error%.

20.5.3.2 Error.prototype.message

The initial value of Error.prototype.message is the empty String.

20.5.3.3 Error.prototype.name

The initial value of Error.prototype.name is"Error".

20.5.3.4 Error.prototype.toString ( )

The following steps are taken:

  1. Let O be thethisvalue.
  2. IfType(O) is not Object, throw aTypeErrorexception.
  3. Let name be ? Get(O,"name").
  4. If name isundefined, set name to"Error"; otherwise set name to ? ToString(name).
  5. Let msg be ? Get(O,"message").
  6. If msg isundefined, set msg to the empty String; otherwise set msg to ? ToString(msg).
  7. If name is the empty String, return msg.
  8. If msg is the empty String, return name.
  9. Return thestring-concatenationof name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.

20.5.4 Properties of Error Instances

Error instances are ordinary objects that inherit properties from theError prototype objectand have an [[ErrorData]] internal slot whose value isundefined. The only specified uses of [[ErrorData]] is to identify Error, AggregateError, and NativeError instances as Error objects within Object.prototype.toString.

20.5.5 Native Error Types Used in This Standard

A new instance of one of the NativeError objects below or of the AggregateError object is thrown when a runtime error is detected. All NativeError objects share the same structure, as described in20.5.6.

20.5.5.1 EvalError

The EvalErrorconstructoris %EvalError%.

This exception is not currently used within this specification. This object remains for compatibility with previous editions of this specification.

20.5.5.2 RangeError

The RangeErrorconstructoris %RangeError%.

Indicates a value that is not in the set or range of allowable values.

20.5.5.3 ReferenceError

The ReferenceErrorconstructoris %ReferenceError%.

Indicate that an invalid reference has been detected.

20.5.5.4 SyntaxError

The SyntaxErrorconstructoris %SyntaxError%.

Indicates that a parsing error has occurred.

20.5.5.5 TypeError

The TypeErrorconstructoris %TypeError%.

TypeError is used to indicate an unsuccessful operation when none of the other NativeError objects are an appropriate indication of the failure cause.

20.5.5.6 URIError

The URIErrorconstructoris %URIError%.

Indicates that one of the global URI handling functions was used in a way that is incompatible with its definition.

20.5.6 NativeError Object Structure

When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the NativeError objects defined in20.5.5. Each of these objects has the structure described below, differing only in the name used as theconstructorname instead of NativeError, in the"name"property of the prototype object, and in theimplementation-defined"message"property of the prototype object.

For each error object, references to NativeError in the definition should be replaced with the appropriate error object name from20.5.5.

20.5.6.1 The NativeError Constructors

Each NativeErrorconstructor:

  • creates and initializes a new NativeError object when called as a function rather than as aconstructor. A call of the object as a function is equivalent to calling it as aconstructorwith the same arguments. Thus the function call NativeError(…) is equivalent to the object creation expression new NativeError(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified NativeError behaviour must include a super call to the NativeErrorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.6.1.1 NativeError ( message )

When a NativeError function is called with argument message, the following steps are taken:

  1. If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget, "%NativeError.prototype%", « [[ErrorData]] »).
  3. If message is notundefined, then
    1. Let msg be ? ToString(message).
    2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
    3. Perform ! DefinePropertyOrThrow(O,"message", msgDesc).
  4. Return O.

The actual value of the string passed in step2is either"%EvalError.prototype%","%RangeError.prototype%","%ReferenceError.prototype%","%SyntaxError.prototype%","%TypeError.prototype%", or"%URIError.prototype%"corresponding to which NativeErrorconstructoris being defined.

20.5.6.2 Properties of the NativeError Constructors

Each NativeErrorconstructor:

  • has a [[Prototype]] internal slot whose value is%Error%.
  • has a"name"property whose value is the String value"NativeError".
  • has the following properties:

20.5.6.2.1 NativeError.prototype

The initial value of NativeError.prototype is a NativeError prototype object (20.5.6.3). Each NativeErrorconstructorhas a distinct prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.5.6.3 Properties of the NativeError Prototype Objects

Each NativeError prototype object:

  • is anordinary object.
  • is not an Error instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Error.prototype%.

20.5.6.3.1 NativeError.prototype.constructor

The initial value of the"constructor"property of the prototype for a given NativeErrorconstructoris the corresponding intrinsic object %NativeError% (20.5.6.1).

20.5.6.3.2 NativeError.prototype.message

The initial value of the"message"property of the prototype for a given NativeErrorconstructoris the empty String.

20.5.6.3.3 NativeError.prototype.name

The initial value of the"name"property of the prototype for a given NativeErrorconstructoris the String value consisting of the name of theconstructor(the name used instead of NativeError).

20.5.6.4 Properties of NativeError Instances

NativeError instances are ordinary objects that inherit properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value isundefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.

20.5.7 AggregateError Objects

20.5.7.1 The AggregateError Constructor

The AggregateErrorconstructor:

  • is %AggregateError%.
  • is the initial value of the"AggregateError"property of theglobal object.
  • creates and initializes a new AggregateError object when called as a function rather than as aconstructor. Thus the function call AggregateError(…) is equivalent to the object creation expression new AggregateError(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AggregateError behaviour must include a super call to the AggregateErrorconstructorto create and initialize subclass instances with an [[ErrorData]] internal slot.

20.5.7.1.1 AggregateError ( errors, message )

When theAggregateErrorfunction is called with arguments errors and message, the following steps are taken:

  1. If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
  2. Let O be ? OrdinaryCreateFromConstructor(newTarget,"%AggregateError.prototype%", « [[ErrorData]] »).
  3. If message is notundefined, then
    1. Let msg be ? ToString(message).
    2. Let msgDesc be the PropertyDescriptor { [[Value]]: msg, [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:true}.
    3. Perform ! DefinePropertyOrThrow(O,"message", msgDesc).
  4. Let errorsList be ? IterableToList(errors).
  5. Perform ! DefinePropertyOrThrow(O,"errors", PropertyDescriptor { [[Configurable]]:true, [[Enumerable]]:false, [[Writable]]:true, [[Value]]: ! CreateArrayFromList(errorsList) }).
  6. Return O.

20.5.7.2 Properties of the AggregateError Constructor

The AggregateErrorconstructor:

  • has a [[Prototype]] internal slot whose value is%Error%.
  • has the following properties:

20.5.7.2.1 AggregateError.prototype

The initial value of AggregateError.prototype is%AggregateError.prototype%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

20.5.7.3 Properties of the AggregateError Prototype Object

The AggregateError prototype object:

  • is %AggregateError.prototype%.
  • is anordinary object.
  • is not an Error instance or an AggregateError instance and does not have an [[ErrorData]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Error.prototype%.

20.5.7.3.1 AggregateError.prototype.constructor

The initial value of AggregateError.prototype.constructor is%AggregateError%.

20.5.7.3.2 AggregateError.prototype.message

The initial value of AggregateError.prototype.message is the empty String.

20.5.7.3.3 AggregateError.prototype.name

The initial value of AggregateError.prototype.name is"AggregateError".

20.5.7.4 Properties of AggregateError Instances

AggregateError instances are ordinary objects that inherit properties from theirAggregateError prototype objectand have an [[ErrorData]] internal slot whose value isundefined. The only specified use of [[ErrorData]] is by Object.prototype.toString (20.1.3.6) to identify Error, AggregateError, or NativeError instances.

21 Numbers and Dates

21.1 Number Objects

21.1.1 The Number Constructor

The Numberconstructor:

  • is %Number%.
  • is the initial value of the"Number"property of theglobal object.
  • creates and initializes a new Number object when called as aconstructor.
  • performs a type conversion when called as a function rather than as aconstructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Number behaviour must include a super call to the Numberconstructorto create and initialize the subclass instance with a [[NumberData]] internal slot.

21.1.1.1 Number ( value )

When Number is called with argument value, the following steps are taken:

  1. If value is present, then
    1. Let prim be ? ToNumeric(value).
    2. IfType(prim) is BigInt, let n be𝔽((prim)).
    3. Otherwise, let n be prim.
  2. Else,
    1. Let n be+0𝔽.
  3. If NewTarget isundefined, return n.
  4. Let O be ? OrdinaryCreateFromConstructor(NewTarget,"%Number.prototype%", « [[NumberData]] »).
  5. Set O.[[NumberData]] to n.
  6. Return O.

21.1.2 Properties of the Number Constructor

The Numberconstructor:

21.1.2.1 Number.EPSILON

The value of Number.EPSILON is theNumber valuefor the magnitude of the difference between 1 and the smallest value greater than 1 that is representable as aNumber value, which is approximately 2.2204460492503130808472633361816 × 10-16.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.2 Number.isFinite ( number )

When Number.isFinite is called with one argument number, the following steps are taken:

  1. IfType(number) is not Number, returnfalse.
  2. If number isNaN,+∞𝔽, or-∞𝔽, returnfalse.
  3. Otherwise, returntrue.

21.1.2.3 Number.isInteger ( number )

When Number.isInteger is called with one argument number, the following steps are taken:

  1. Return ! IsIntegralNumber(number).

21.1.2.4 Number.isNaN ( number )

When Number.isNaN is called with one argument number, the following steps are taken:

  1. IfType(number) is not Number, returnfalse.
  2. If number isNaN, returntrue.
  3. Otherwise, returnfalse.
Note

This function differs from the global isNaN function (19.2.3) in that it does not convert its argument to a Number before determining whether it isNaN.

21.1.2.5 Number.isSafeInteger ( number )

When Number.isSafeInteger is called with one argument number, the following steps are taken:

  1. If ! IsIntegralNumber(number) istrue, then
    1. Ifabs((number)) ≤ 253 - 1, returntrue.
  2. Returnfalse.

21.1.2.6 Number.MAX_SAFE_INTEGER

Note

The value of Number.MAX_SAFE_INTEGER is the largestintegral Numbern such that(n) and(n) + 1 are both exactly representable as aNumber value.

The value of Number.MAX_SAFE_INTEGER is9007199254740991𝔽 (𝔽(253 - 1)).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.7 Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which is approximately1.7976931348623157 × 10308.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.8 Number.MIN_SAFE_INTEGER

Note

The value of Number.MIN_SAFE_INTEGER is the smallestintegral Numbern such that(n) and(n) - 1 are both exactly representable as aNumber value.

The value of Number.MIN_SAFE_INTEGER is-9007199254740991𝔽 (𝔽(-(253 - 1))).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.9 Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately5 × 10-324.

In theIEEE 754-2019double precision binary representation, the smallest possible value is a denormalized number. If an implementation does not support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero positive value that can actually be represented by the implementation.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.10 Number.NaN

The value of Number.NaN isNaN.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.11 Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is-∞𝔽.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.12 Number.parseFloat ( string )

The value of the Number.parseFloatdata propertyis the same built-infunction objectthat is the initial value of the"parseFloat"property of theglobal objectdefined in19.2.4.

21.1.2.13 Number.parseInt ( string, radix )

The value of the Number.parseIntdata propertyis the same built-infunction objectthat is the initial value of the"parseInt"property of theglobal objectdefined in19.2.5.

21.1.2.14 Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is+∞𝔽.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.2.15 Number.prototype

The initial value of Number.prototype is theNumber prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.1.3 Properties of the Number Prototype Object

The Number prototype object:

  • is %Number.prototype%.
  • is anordinary object.
  • is itself a Number object; it has a [[NumberData]] internal slot with the value+0𝔽.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not generic and thethisvalue passed to them must be either aNumber valueor an object that has a [[NumberData]] internal slot that has been initialized to aNumber value.

The abstract operation thisNumberValue takes argument value. It performs the following steps when called:

  1. IfType(value) is Number, return value.
  2. IfType(value) is Object and value has a [[NumberData]] internal slot, then
    1. Let n be value.[[NumberData]].
    2. Assert:Type(n) is Number.
    3. Return n.
  3. Throw aTypeErrorexception.

The phrase “thisNumber value” within the specification of a method refers to the result returned by calling the abstract operationthisNumberValuewith thethisvalue of the method invocation passed as the argument.

21.1.3.1 Number.prototype.constructor

The initial value of Number.prototype.constructor is%Number%.

21.1.3.2 Number.prototype.toExponential ( fractionDigits )

Return a String containing thisNumber valuerepresented in decimal exponential notation with one digit before the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits isundefined, include as many significand digits as necessary to uniquely specify the Number (just like inToStringexcept that in this case the Number is always output in exponential notation). Specifically, perform the following steps:

  1. Let x be ? thisNumberValue(thisvalue).
  2. Let f be ? ToIntegerOrInfinity(fractionDigits).
  3. Assert: If fractionDigits isundefined, then f is 0.
  4. If x is not finite, return !Number::toString(x).
  5. If f < 0 or f > 100, throw aRangeErrorexception.
  6. Set x to(x).
  7. Let s be the empty String.
  8. If x < 0, then
    1. Set s to"-".
    2. Set x to -x.
  9. If x = 0, then
    1. Let m be the String value consisting of f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Let e be 0.
  10. Else,
    1. If fractionDigits is notundefined, then
      1. Let e and n be integers such that 10fn < 10f + 1 and for which n × 10e - f - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - f is larger.
    2. Else,
      1. Let e, n, and f be integers such that f ≥ 0, 10fn < 10f + 1,𝔽(n × 10e - f) is𝔽(x), and f is as small as possible. Note that the decimal representation of n has f + 1 digits, n is not divisible by 10, and the least significant digit of n is not necessarily uniquely determined by these criteria.
    3. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
  11. If f ≠ 0, then
    1. Let a be the first code unit of m.
    2. Let b be the other f code units of m.
    3. Set m to thestring-concatenationof a,".", and b.
  12. If e = 0, then
    1. Let c be"+".
    2. Let d be"0".
  13. Else,
    1. If e > 0, let c be"+".
    2. Else,
      1. Assert: e < 0.
      2. Let c be"-".
      3. Set e to -e.
    3. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
  14. Set m to thestring-concatenationof m,"e", c, and d.
  15. Return thestring-concatenationof s and m.
Note

For implementations that provide more accurate conversions than required by the rules above, it is recommended that the following alternative version of step10.b.ibe used as a guideline:

  1. Let e, n, and f be integers such that f ≥ 0, 10fn < 10f + 1,𝔽(n × 10e - f) is𝔽(x), and f is as small as possible. If there are multiple possibilities for n, choose the value of n for which𝔽(n × 10e - f) is closest in value to𝔽(x). If there are two such possible values of n, choose the one that is even.

21.1.3.3 Number.prototype.toFixed ( fractionDigits )

Note 1

toFixed returns a String containing thisNumber valuerepresented in decimal fixed-point notation with fractionDigits digits after the decimal point. If fractionDigits isundefined, 0 is assumed.

The following steps are performed:

  1. Let x be ? thisNumberValue(thisvalue).
  2. Let f be ? ToIntegerOrInfinity(fractionDigits).
  3. Assert: If fractionDigits isundefined, then f is 0.
  4. If f is not finite, throw aRangeErrorexception.
  5. If f < 0 or f > 100, throw aRangeErrorexception.
  6. If x is not finite, return !Number::toString(x).
  7. Set x to(x).
  8. Let s be the empty String.
  9. If x < 0, then
    1. Set s to"-".
    2. Set x to -x.
  10. If x ≥ 1021, then
    1. Let m be ! ToString(𝔽(x)).
  11. Else,
    1. Let n be anintegerfor which n / 10f - x is as close to zero as possible. If there are two such n, pick the larger n.
    2. If n = 0, let m be the String"0". Otherwise, let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If f ≠ 0, then
      1. Let k be the length of m.
      2. If kf, then
        1. Let z be the String value consisting of f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
        2. Set m to thestring-concatenationof z and m.
        3. Set k to f + 1.
      3. Let a be the first k - f code units of m.
      4. Let b be the other f code units of m.
      5. Set m to thestring-concatenationof a,".", and b.
  12. Return thestring-concatenationof s and m.
Note 2

The output of toFixed may be more precise than toString for some values because toString only prints enough significant digits to distinguish the number from adjacent Number values. For example,

(1000000000000000128).toString() returns"1000000000000000100", while
(1000000000000000128).toFixed(0) returns"1000000000000000128".

21.1.3.4 Number.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Number.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Produces a String value that represents thisNumber valueformatted according to the conventions of thehost environment's current locale. This function isimplementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.1.3.5 Number.prototype.toPrecision ( precision )

Return a String containing thisNumber valuerepresented either in decimal exponential notation with one digit before the significand's decimal point andprecision - 1digits after the significand's decimal point or in decimal fixed notation with precision significant digits. If precision isundefined, callToStringinstead. Specifically, perform the following steps:

  1. Let x be ? thisNumberValue(thisvalue).
  2. If precision isundefined, return ! ToString(x).
  3. Let p be ? ToIntegerOrInfinity(precision).
  4. If x is not finite, return !Number::toString(x).
  5. If p < 1 or p > 100, throw aRangeErrorexception.
  6. Set x to(x).
  7. Let s be the empty String.
  8. If x < 0, then
    1. Set s to the code unit 0x002D (HYPHEN-MINUS).
    2. Set x to -x.
  9. If x = 0, then
    1. Let m be the String value consisting of p occurrences of the code unit 0x0030 (DIGIT ZERO).
    2. Let e be 0.
  10. Else,
    1. Let e and n be integers such that 10p - 1n < 10p and for which n × 10e - p + 1 - x is as close to zero as possible. If there are two such sets of e and n, pick the e and n for which n × 10e - p + 1 is larger.
    2. Let m be the String value consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
    3. If e < -6 or ep, then
      1. Assert: e ≠ 0.
      2. If p ≠ 1, then
        1. Let a be the first code unit of m.
        2. Let b be the other p - 1 code units of m.
        3. Set m to thestring-concatenationof a,".", and b.
      3. If e > 0, then
        1. Let c be the code unit 0x002B (PLUS SIGN).
      4. Else,
        1. Assert: e < 0.
        2. Let c be the code unit 0x002D (HYPHEN-MINUS).
        3. Set e to -e.
      5. Let d be the String value consisting of the digits of the decimal representation of e (in order, with no leading zeroes).
      6. Return thestring-concatenationof s, m, the code unit 0x0065 (LATIN SMALL LETTER E), c, and d.
  11. If e = p - 1, return thestring-concatenationof s and m.
  12. If e ≥ 0, then
    1. Set m to thestring-concatenationof the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and the remaining p - (e + 1) code units of m.
  13. Else,
    1. Set m to thestring-concatenationof the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
  14. Return thestring-concatenationof s and m.

21.1.3.6 Number.prototype.toString ( [ radix ] )

Note

The optional radix should be anintegral Numbervalue in the inclusive range2𝔽 to36𝔽. If radix isundefinedthen10𝔽 is used as the value of radix.

The following steps are performed:

  1. Let x be ? thisNumberValue(thisvalue).
  2. If radix isundefined, let radixMV be 10.
  3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
  4. If radixMV < 2 or radixMV > 36, throw aRangeErrorexception.
  5. If radixMV = 10, return ! ToString(x).
  6. Return the String representation of thisNumber valueusing the radix specified by radixMV. Letters a-z are used for digits with values 10 through 35. The precise algorithm isimplementation-defined, however the algorithm should be a generalization of that specified in6.1.6.1.20.

The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a Number or a Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

The"length"property of the toString method is1𝔽.

21.1.3.7 Number.prototype.valueOf ( )

  1. Return ? thisNumberValue(thisvalue).

21.1.4 Properties of Number Instances

Number instances are ordinary objects that inherit properties from theNumber prototype object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is theNumber valuerepresented by this Number object.

21.2 BigInt Objects

21.2.1 The BigInt Constructor

The BigIntconstructor:

  • is %BigInt%.
  • is the initial value of the"BigInt"property of theglobal object.
  • performs a type conversion when called as a function rather than as aconstructor.
  • is not intended to be used with the new operator or to be subclassed. It may be used as the value of an extends clause of a class definition but a super call to the BigIntconstructorwill cause an exception.

21.2.1.1 BigInt ( value )

When BigInt is called with argument value, the following steps are taken:

  1. If NewTarget is notundefined, throw aTypeErrorexception.
  2. Let prim be ? ToPrimitive(value,number).
  3. IfType(prim) is Number, return ? NumberToBigInt(prim).
  4. Otherwise, return ? ToBigInt(value).

21.2.1.1.1 NumberToBigInt ( number )

The abstract operation NumberToBigInt takes argument number (a Number). It performs the following steps when called:

  1. IfIsIntegralNumber(number) isfalse, throw aRangeErrorexception.
  2. Return the BigInt value that represents(number).

21.2.2 Properties of the BigInt Constructor

The value of the [[Prototype]] internal slot of the BigIntconstructoris%Function.prototype%.

The BigIntconstructorhas the following properties:

21.2.2.1 BigInt.asIntN ( bits, bigint )

When the BigInt.asIntN function is called with two arguments bits and bigint, the following steps are taken:

  1. Set bits to ? ToIndex(bits).
  2. Set bigint to ? ToBigInt(bigint).
  3. Let mod be(bigint)modulo2bits.
  4. If mod ≥ 2bits - 1, return(mod - 2bits); otherwise, return(mod).

21.2.2.2 BigInt.asUintN ( bits, bigint )

When the BigInt.asUintN function is called with two arguments bits and bigint, the following steps are taken:

  1. Set bits to ? ToIndex(bits).
  2. Set bigint to ? ToBigInt(bigint).
  3. Return the BigInt value that represents(bigint)modulo2bits.

21.2.2.3 BigInt.prototype

The initial value of BigInt.prototype is theBigInt prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.2.3 Properties of the BigInt Prototype Object

The BigInt prototype object:

  • is %BigInt.prototype%.
  • is anordinary object.
  • is not a BigInt object; it does not have a [[BigIntData]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

The abstract operation thisBigIntValue takes argument value. It performs the following steps when called:

  1. IfType(value) is BigInt, return value.
  2. IfType(value) is Object and value has a [[BigIntData]] internal slot, then
    1. Assert:Type(value.[[BigIntData]]) is BigInt.
    2. Return value.[[BigIntData]].
  3. Throw aTypeErrorexception.

The phrase “this BigInt value” within the specification of a method refers to the result returned by calling the abstract operationthisBigIntValuewith thethisvalue of the method invocation passed as the argument.

21.2.3.1 BigInt.prototype.constructor

The initial value of BigInt.prototype.constructor is%BigInt%.

21.2.3.2 BigInt.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the BigInt.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Produces a String value that represents this BigInt value formatted according to the conventions of thehost environment's current locale. This function isimplementation-defined, and it is permissible, but not encouraged, for it to return the same thing as toString.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.2.3.3 BigInt.prototype.toString ( [ radix ] )

Note

The optional radix should be anintegral Numbervalue in the inclusive range2𝔽 to36𝔽. If radix isundefinedthen10𝔽 is used as the value of radix.

The following steps are performed:

  1. Let x be ? thisBigIntValue(thisvalue).
  2. If radix isundefined, let radixMV be 10.
  3. Else, let radixMV be ? ToIntegerOrInfinity(radix).
  4. If radixMV < 2 or radixMV > 36, throw aRangeErrorexception.
  5. If radixMV = 10, return ! ToString(x).
  6. Return the String representation of thisNumber valueusing the radix specified by radixMV. Letters a-z are used for digits with values 10 through 35. The precise algorithm isimplementation-defined, however the algorithm should be a generalization of that specified in6.1.6.2.23.

The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

21.2.3.4 BigInt.prototype.valueOf ( )

  1. Return ? thisBigIntValue(thisvalue).

21.2.3.5 BigInt.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"BigInt".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

21.3 The Math Object

The Math object:

  • is %Math%.
  • is the initial value of the"Math"property of theglobal object.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is not afunction object.
  • does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.
Note

In this specification, the phrase “theNumber valuefor x” has a technical meaning defined in6.1.6.1.

21.3.1 Value Properties of the Math Object

21.3.1.1 Math.E

TheNumber valuefor e, the base of the natural logarithms, which is approximately 2.7182818284590452354.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.3.1.2 Math.LN10

TheNumber valuefor the natural logarithm of 10, which is approximately 2.302585092994046.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.3.1.3 Math.LN2

TheNumber valuefor the natural logarithm of 2, which is approximately 0.6931471805599453.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.3.1.4 Math.LOG10E

TheNumber valuefor the base-10 logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

Note

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

21.3.1.5 Math.LOG2E

TheNumber valuefor the base-2 logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

Note

The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

21.3.1.6 Math.PI

TheNumber valuefor π, the ratio of the circumference of a circle to its diameter, which is approximately 3.1415926535897932.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.3.1.7 Math.SQRT1_2

TheNumber valuefor the square root of ½, which is approximately 0.7071067811865476.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

Note

The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

21.3.1.8 Math.SQRT2

TheNumber valuefor the square root of 2, which is approximately 1.4142135623730951.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.3.1.9 Math [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Math".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

21.3.2 Function Properties of the Math Object

Note

The behaviour of the functions acos, acosh, asin, asinh, atan, atanh, atan2, cbrt, cos, cosh, exp, expm1, hypot, log, log1p, log2, log10, pow, random, sin, sinh, sqrt, tan, and tanh is not precisely specified here except to require specific results for certain argument values that represent boundary cases of interest. For other argument values, these functions are intended to compute approximations to the results of familiar mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that is available to C programmers on that platform.

Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that implementations use the approximation algorithms forIEEE 754-2019arithmetic contained in fdlibm, the freely distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

21.3.2.1 Math.abs ( x )

Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

When the Math.abs method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, returnNaN.
  3. If n is-0𝔽, return+0𝔽.
  4. If n is-∞𝔽, return+∞𝔽.
  5. If n <+0𝔽, return -n.
  6. Return n.

21.3.2.2 Math.acos ( x )

Returns the inverse cosine of x. The result is expressed in radians and ranges from+0𝔽 to𝔽(π), inclusive.

When the Math.acos method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n >1𝔽, or n <-1𝔽, returnNaN.
  3. If n is1𝔽, return+0𝔽.
  4. Return animplementation-approximatedvalue representing the result of the inverse cosine of(n).

21.3.2.3 Math.acosh ( x )

Returns the inverse hyperbolic cosine of x.

When the Math.acosh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaNor n is+∞𝔽, return n.
  3. If n is1𝔽, return+0𝔽.
  4. If n <1𝔽, returnNaN.
  5. Return animplementation-approximatedvalue representing the result of the inverse hyperbolic cosine of(n).

21.3.2.4 Math.asin ( x )

Returns the inverse sine of x. The result is expressed in radians and ranges from𝔽(-π / 2) to𝔽(π / 2), inclusive.

When the Math.asin method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n >1𝔽 or n <-1𝔽, returnNaN.
  4. Return animplementation-approximatedvalue representing the result of the inverse sine of(n).

21.3.2.5 Math.asinh ( x )

Returns the inverse hyperbolic sine of x.

When the Math.asinh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. Return animplementation-approximatedvalue representing the result of the inverse hyperbolic sine of(n).

21.3.2.6 Math.atan ( x )

Returns the inverse tangent of x. The result is expressed in radians and ranges from𝔽(-π / 2) to𝔽(π / 2), inclusive.

When the Math.atan method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n is+∞𝔽, return animplementation-approximatedvalue representing π / 2.
  4. If n is-∞𝔽, return animplementation-approximatedvalue representing -π / 2.
  5. Return animplementation-approximatedvalue representing the result of the inverse tangent of(n).

21.3.2.7 Math.atanh ( x )

Returns the inverse hyperbolic tangent of x.

When the Math.atanh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n >1𝔽 or n <-1𝔽, returnNaN.
  4. If n is1𝔽, return+∞𝔽.
  5. If n is-1𝔽, return-∞𝔽.
  6. Return animplementation-approximatedvalue representing the result of the inverse hyperbolic tangent of(n).

21.3.2.8 Math.atan2 ( y, x )

Returns the inverse tangent of the quotienty / xof the arguments y and x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional and traditional for the two-argument inverse tangent function that the argument named y be first and the argument named x be second. The result is expressed in radians and ranges from -π to +π, inclusive.

When the Math.atan2 method is called with arguments y and x, the following steps are taken:

  1. Let ny be ? ToNumber(y).
  2. Let nx be ? ToNumber(x).
  3. If ny isNaNor nx isNaN, returnNaN.
  4. If ny is+∞𝔽, then
    1. If nx is+∞𝔽, return animplementation-approximatedvalue representing π / 4.
    2. If nx is-∞𝔽, return animplementation-approximatedvalue representing 3π / 4.
    3. Return animplementation-approximatedvalue representing π / 2.
  5. If ny is-∞𝔽, then
    1. If nx is+∞𝔽, return animplementation-approximatedvalue representing -π / 4.
    2. If nx is-∞𝔽, return animplementation-approximatedvalue representing -3π / 4.
    3. Return animplementation-approximatedvalue representing -π / 2.
  6. If ny is+0𝔽, then
    1. If nx >+0𝔽 or nx is+0𝔽, return+0𝔽.
    2. Return animplementation-approximatedvalue representing π.
  7. If ny is-0𝔽, then
    1. If nx >+0𝔽 or nx is+0𝔽, return-0𝔽.
    2. Return animplementation-approximatedvalue representing -π.
  8. Assert: ny is finite and is neither+0𝔽 nor-0𝔽.
  9. If ny >+0𝔽, then
    1. If nx is+∞𝔽, return+0𝔽.
    2. If nx is-∞𝔽, return animplementation-approximatedvalue representing π.
    3. If nx is+0𝔽 or nx is-0𝔽, return animplementation-approximatedvalue representing π / 2.
  10. If ny <+0𝔽, then
    1. If nx is+∞𝔽, return-0𝔽.
    2. If nx is-∞𝔽, return animplementation-approximatedvalue representing -π.
    3. If nx is+0𝔽 or nx is-0𝔽, return animplementation-approximatedvalue representing -π / 2.
  11. Assert: nx is finite and is neither+0𝔽 nor-0𝔽.
  12. Return animplementation-approximatedvalue representing the result of the inverse tangent of the quotient(ny) /(nx).

21.3.2.9 Math.cbrt ( x )

Returns the cube root of x.

When the Math.cbrt method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. Return animplementation-approximatedvalue representing the result of the cube root of(n).

21.3.2.10 Math.ceil ( x )

Returns the smallest (closest to -∞)integral Numbervalue that is not less than x. If x is already anintegral Number, the result is x.

When the Math.ceil method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. If n <+0𝔽 and n >-1𝔽, return-0𝔽.
  4. If n is anintegral Number, return n.
  5. Return the smallest (closest to -∞)integral Numbervalue that is not less than n.
Note

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).

21.3.2.11 Math.clz32 ( x )

When the Math.clz32 method is called with argument x, the following steps are taken:

  1. Let n be ? ToUint32(x).
  2. Let p be the number of leading zero bits in the unsigned 32-bit binary representation of n.
  3. Return𝔽(p).
Note

If n is+0𝔽 or n is-0𝔽, this method returns32𝔽. If the most significant bit of the 32-bit binary encoding of n is 1, this method returns+0𝔽.

21.3.2.12 Math.cos ( x )

Returns the cosine of x. The argument is expressed in radians.

When the Math.cos method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n is+∞𝔽 or n is-∞𝔽, returnNaN.
  4. Return animplementation-approximatedvalue representing the result of the cosine of(n).

21.3.2.13 Math.cosh ( x )

Returns the hyperbolic cosine of x.

When the Math.cosh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+∞𝔽, or n is-∞𝔽, return n.
  3. If n is+0𝔽 or n is-0𝔽, return1𝔽.
  4. Return animplementation-approximatedvalue representing the result of the hyperbolic cosine of(n).
Note

The value of Math.cosh(x) is the same as the value of (Math.exp(x) + Math.exp(-x)) / 2.

21.3.2.14 Math.exp ( x )

Returns the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms).

When the Math.exp method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaNor n is+∞𝔽, return n.
  3. If n is+0𝔽 or n is-0𝔽, return1𝔽.
  4. If n is-∞𝔽, return+0𝔽.
  5. Return animplementation-approximatedvalue representing the result of the exponential function of(n).

21.3.2.15 Math.expm1 ( x )

Returns the result of subtracting 1 from the exponential function of x (e raised to the power of x, where e is the base of the natural logarithms). The result is computed in a way that is accurate even when the value of x is close to 0.

When the Math.expm1 method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, or n is+∞𝔽, return n.
  3. If n is-∞𝔽, return-1𝔽.
  4. Return animplementation-approximatedvalue representing the result of subtracting 1 from the exponential function of(n).

21.3.2.16 Math.floor ( x )

Returns the greatest (closest to +∞)integral Numbervalue that is not greater than x. If x is already anintegral Number, the result is x.

When the Math.floor method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. If n <1𝔽 and n >+0𝔽, return+0𝔽.
  4. If n is anintegral Number, return n.
  5. Return the greatest (closest to +∞)integral Numbervalue that is not greater than n.
Note

The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

21.3.2.17 Math.fround ( x )

When the Math.fround method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, returnNaN.
  3. If n is one of+0𝔽,-0𝔽,+∞𝔽, or-∞𝔽, return n.
  4. Let n32 be the result of converting n to a value inIEEE 754-2019binary32 format using roundTiesToEven mode.
  5. Let n64 be the result of converting n32 to a value inIEEE 754-2019binary64 format.
  6. Return the ECMAScriptNumber valuecorresponding to n64.

21.3.2.18 Math.hypot ( ...args )

Returns the square root of the sum of squares of its arguments.

When the Math.hypot method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:

  1. Let coerced be a new emptyList.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. For each element number of coerced, do
    1. If number is+∞𝔽 or number is-∞𝔽, return+∞𝔽.
  4. Let onlyZero betrue.
  5. For each element number of coerced, do
    1. If number isNaN, returnNaN.
    2. If number is neither+0𝔽 nor-0𝔽, set onlyZero tofalse.
  6. If onlyZero istrue, return+0𝔽.
  7. Return animplementation-approximatedvalue representing the square root of the sum of squares of the mathematical values of the elements of coerced.

The"length"property of the hypot method is2𝔽.

Note

Implementations should take care to avoid the loss of precision from overflows and underflows that are prone to occur in naive implementations when this function is called with two or more arguments.

21.3.2.19 Math.imul ( x, y )

When Math.imul is called with arguments x and y, the following steps are taken:

  1. Let a be(?ToUint32(x)).
  2. Let b be(?ToUint32(y)).
  3. Let product be (a × b)modulo232.
  4. If product ≥ 231, return𝔽(product - 232); otherwise return𝔽(product).

21.3.2.20 Math.log ( x )

Returns the natural logarithm of x.

When the Math.log method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaNor n is+∞𝔽, return n.
  3. If n is1𝔽, return+0𝔽.
  4. If n is+0𝔽 or n is-0𝔽, return-∞𝔽.
  5. If n <+0𝔽, returnNaN.
  6. Return animplementation-approximatedvalue representing the result of the natural logarithm of(n).

21.3.2.21 Math.log1p ( x )

Returns the natural logarithm of 1 + x. The result is computed in a way that is accurate even when the value of x is close to zero.

When the Math.log1p method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, or n is+∞𝔽, return n.
  3. If n is-1𝔽, return-∞𝔽.
  4. If n <-1𝔽, returnNaN.
  5. Return animplementation-approximatedvalue representing the result of the natural logarithm of 1 +(n).

21.3.2.22 Math.log10 ( x )

Returns the base 10 logarithm of x.

When the Math.log10 method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaNor n is+∞𝔽, return n.
  3. If n is1𝔽, return+0𝔽.
  4. If n is+0𝔽 or n is-0𝔽, return-∞𝔽.
  5. If n <+0𝔽, returnNaN.
  6. Return animplementation-approximatedvalue representing the result of the base 10 logarithm of(n).

21.3.2.23 Math.log2 ( x )

Returns the base 2 logarithm of x.

When the Math.log2 method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaNor n is+∞𝔽, return n.
  3. If n is1𝔽, return+0𝔽.
  4. If n is+0𝔽 or n is-0𝔽, return-∞𝔽.
  5. If n <+0𝔽, returnNaN.
  6. Return animplementation-approximatedvalue representing the result of the base 2 logarithm of(n).

21.3.2.24 Math.max ( ...args )

Given zero or more arguments, callsToNumberon each of the arguments and returns the largest of the resulting values.

When the Math.max method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:

  1. Let coerced be a new emptyList.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. Let highest be-∞𝔽.
  4. For each element number of coerced, do
    1. If number isNaN, returnNaN.
    2. If number is+0𝔽 and highest is-0𝔽, set highest to+0𝔽.
    3. If number > highest, set highest to number.
  5. Return highest.
Note

The comparison of values to determine the largest value is done using theIsLessThanalgorithm except that+0𝔽 is considered to be larger than-0𝔽.

The"length"property of the max method is2𝔽.

21.3.2.25 Math.min ( ...args )

Given zero or more arguments, callsToNumberon each of the arguments and returns the smallest of the resulting values.

When the Math.min method is called with zero or more arguments which form the rest parameter ...args, the following steps are taken:

  1. Let coerced be a new emptyList.
  2. For each element arg of args, do
    1. Let n be ? ToNumber(arg).
    2. Append n to coerced.
  3. Let lowest be+∞𝔽.
  4. For each element number of coerced, do
    1. If number isNaN, returnNaN.
    2. If number is-0𝔽 and lowest is+0𝔽, set lowest to-0𝔽.
    3. If number < lowest, set lowest to number.
  5. Return lowest.
Note

The comparison of values to determine the largest value is done using theIsLessThanalgorithm except that+0𝔽 is considered to be larger than-0𝔽.

The"length"property of the min method is2𝔽.

21.3.2.26 Math.pow ( base, exponent )

When the Math.pow method is called with arguments base and exponent, the following steps are taken:

  1. Set base to ? ToNumber(base).
  2. Set exponent to ? ToNumber(exponent).
  3. Return !Number::exponentiate(base, exponent).

21.3.2.27 Math.random ( )

Returns aNumber valuewith positive sign, greater than or equal to+0𝔽 but strictly less than1𝔽, chosen randomly or pseudo randomly with approximately uniform distribution over that range, using animplementation-definedalgorithm or strategy. This function takes no arguments.

Each Math.random function created for distinct realms must produce a distinct sequence of values from successive calls.

21.3.2.28 Math.round ( x )

Returns theNumber valuethat is closest to x and is integral. If two integral Numbers are equally close to x, then the result is theNumber valuethat is closer to +∞. If x is already integral, the result is x.

When the Math.round method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN,+∞𝔽,-∞𝔽, or anintegral Number, return n.
  3. If n <0.5𝔽 and n >+0𝔽, return+0𝔽.
  4. If n <+0𝔽 and n-0.5𝔽, return-0𝔽.
  5. Return theintegral Numberclosest to n, preferring the Number closer to +∞ in the case of a tie.
Note 1

Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.

Note 2

The value of Math.round(x) is not always the same as the value of Math.floor(x + 0.5). When x is-0𝔽 or is less than+0𝔽 but greater than or equal to-0.5𝔽, Math.round(x) returns-0𝔽, but Math.floor(x + 0.5) returns+0𝔽. Math.round(x) may also differ from the value of Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.

21.3.2.29 Math.sign ( x )

Returns the sign of x, indicating whether x is positive, negative, or zero.

When the Math.sign method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n <+0𝔽, return-1𝔽.
  4. Return1𝔽.

21.3.2.30 Math.sin ( x )

Returns the sine of x. The argument is expressed in radians.

When the Math.sin method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n is+∞𝔽 or n is-∞𝔽, returnNaN.
  4. Return animplementation-approximatedvalue representing the result of the sine of(n).

21.3.2.31 Math.sinh ( x )

Returns the hyperbolic sine of x.

When the Math.sinh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. Return animplementation-approximatedvalue representing the result of the hyperbolic sine of(n).
Note

The value of Math.sinh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / 2.

21.3.2.32 Math.sqrt ( x )

Returns the square root of x.

When the Math.sqrt method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, or n is+∞𝔽, return n.
  3. If n <+0𝔽, returnNaN.
  4. Return animplementation-approximatedvalue representing the result of the square root of(n).

21.3.2.33 Math.tan ( x )

Returns the tangent of x. The argument is expressed in radians.

When the Math.tan method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n is+∞𝔽, or n is-∞𝔽, returnNaN.
  4. Return animplementation-approximatedvalue representing the result of the tangent of(n).

21.3.2.34 Math.tanh ( x )

Returns the hyperbolic tangent of x.

When the Math.tanh method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, or n is-0𝔽, return n.
  3. If n is+∞𝔽, return1𝔽.
  4. If n is-∞𝔽, return-1𝔽.
  5. Return animplementation-approximatedvalue representing the result of the hyperbolic tangent of(n).
Note

The value of Math.tanh(x) is the same as the value of (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) + Math.exp(-x)).

21.3.2.35 Math.trunc ( x )

Returns the integral part of the number x, removing any fractional digits. If x is already integral, the result is x.

When the Math.trunc method is called with argument x, the following steps are taken:

  1. Let n be ? ToNumber(x).
  2. If n isNaN, n is+0𝔽, n is-0𝔽, n is+∞𝔽, or n is-∞𝔽, return n.
  3. If n <1𝔽 and n >+0𝔽, return+0𝔽.
  4. If n <+0𝔽 and n >-1𝔽, return-0𝔽.
  5. Return theintegral Numbernearest n in the direction of+0𝔽.

21.4 Date Objects

21.4.1 Overview of Date Objects and Definitions of Abstract Operations

The followingabstract operationsoperate on time values (defined in21.4.1.1). Note that, in every case, if any argument to one of these functions isNaN, the result will beNaN.

21.4.1.1 Time Values and Time Range

Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly 86,400 seconds (each of which is 1000 milliseconds long).

An ECMAScript time value is a Number, either a finiteintegral Numberrepresenting an instant in time to millisecond precision orNaNrepresenting no specific instant. A time value that is a multiple of24 × 60 × 60 × 1000 = 86,400,000(i.e., is equal to 86,400,000 × d for someintegerd) represents the instant at the start of the UTC day that follows the epoch by d whole UTC days (preceding the epoch for negative d). Every other finite time value t is defined relative to the greatest preceding time value s that is such a multiple, and represents the instant that occurs within the same UTC day as s but follows it by ts milliseconds.

Time values do not account for UTC leap seconds—there are no time values representing instants within positive leap seconds, and there are time values representing instants removed from the UTC timeline by negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.

A Number can exactly represent all integers from -9,007,199,254,740,992 to 9,007,199,254,740,992 (21.1.2.8and21.1.2.6). A time value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds. This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to midnight at the beginning of 1 January 1970 UTC.

The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value+0𝔽.

Note

The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative to 1970. The smaller range supported by a time value as specified in this section is approximately -273,790 to 273,790 years relative to 1970.

21.4.1.2 Day Number and Time within Day

A giventime valuet belongs to day number

Day(t) =𝔽(floor((t /msPerDay)))

where the number of milliseconds per day is

msPerDay =86400000𝔽

The remainder is called the time within the day:

TimeWithinDay(t) =𝔽((t)modulo(msPerDay))

21.4.1.3 Year Number

ECMAScript uses a proleptic Gregorian calendar to map a day number to a year number and to determine the month and date within that year. In this calendar, leap years are precisely those which are (divisible by 4) and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by

DaysInYear(y)
=365𝔽 if ((y)modulo4) ≠ 0
=366𝔽 if ((y)modulo4) = 0 and ((y)modulo100) ≠ 0
=365𝔽 if ((y)modulo100) = 0 and ((y)modulo400) ≠ 0
=366𝔽 if ((y)modulo400) = 0

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day in February. The day number of the first day of year y is given by:

DayFromYear(y) =𝔽(365 × ((y) - 1970) +floor(((y) - 1969) / 4) -floor(((y) - 1901) / 100) +floor(((y) - 1601) / 400))

Thetime valueof the start of a year is:

TimeFromYear(y) =msPerDay×DayFromYear(y)

Atime valuedetermines a year by:

YearFromTime(t) = the largestintegral Numbery (closest to +∞) such thatTimeFromYear(y) ≤ t

The leap-year function is1𝔽 for a time within a leap year and otherwise is+0𝔽:

InLeapYear(t)
=+0𝔽 ifDaysInYear(YearFromTime(t)) =365𝔽
=1𝔽 ifDaysInYear(YearFromTime(t)) =366𝔽

21.4.1.4 Month Number

Months are identified by anintegral Numberin the range+0𝔽 to11𝔽, inclusive. The mappingMonthFromTime(t) from atime valuet to a month number is defined by:

MonthFromTime(t)
=+0𝔽 if+0𝔽DayWithinYear(t) <31𝔽
=1𝔽 if31𝔽DayWithinYear(t) <59𝔽 +InLeapYear(t)
=2𝔽 if59𝔽 +InLeapYear(t) ≤DayWithinYear(t) <90𝔽 +InLeapYear(t)
=3𝔽 if90𝔽 +InLeapYear(t) ≤DayWithinYear(t) <120𝔽 +InLeapYear(t)
=4𝔽 if120𝔽 +InLeapYear(t) ≤DayWithinYear(t) <151𝔽 +InLeapYear(t)
=5𝔽 if151𝔽 +InLeapYear(t) ≤DayWithinYear(t) <181𝔽 +InLeapYear(t)
=6𝔽 if181𝔽 +InLeapYear(t) ≤DayWithinYear(t) <212𝔽 +InLeapYear(t)
=7𝔽 if212𝔽 +InLeapYear(t) ≤DayWithinYear(t) <243𝔽 +InLeapYear(t)
=8𝔽 if243𝔽 +InLeapYear(t) ≤DayWithinYear(t) <273𝔽 +InLeapYear(t)
=9𝔽 if273𝔽 +InLeapYear(t) ≤DayWithinYear(t) <304𝔽 +InLeapYear(t)
=10𝔽 if304𝔽 +InLeapYear(t) ≤DayWithinYear(t) <334𝔽 +InLeapYear(t)
=11𝔽 if334𝔽 +InLeapYear(t) ≤DayWithinYear(t) <365𝔽 +InLeapYear(t)

where

DayWithinYear(t) =Day(t) -DayFromYear(YearFromTime(t))

A month value of+0𝔽 specifies January;1𝔽 specifies February;2𝔽 specifies March;3𝔽 specifies April;4𝔽 specifies May;5𝔽 specifies June;6𝔽 specifies July;7𝔽 specifies August;8𝔽 specifies September;9𝔽 specifies October;10𝔽 specifies November; and11𝔽 specifies December. Note thatMonthFromTime(+0𝔽) =+0𝔽, corresponding to Thursday, 1 January 1970.

21.4.1.5 Date Number

A date number is identified by anintegral Numberin the range1𝔽 through31𝔽, inclusive. The mapping DateFromTime(t) from atime valuet to a date number is defined by:

DateFromTime(t)
=DayWithinYear(t) +1𝔽 ifMonthFromTime(t) =+0𝔽
=DayWithinYear(t) -30𝔽 ifMonthFromTime(t) =1𝔽
=DayWithinYear(t) -58𝔽 -InLeapYear(t) ifMonthFromTime(t) =2𝔽
=DayWithinYear(t) -89𝔽 -InLeapYear(t) ifMonthFromTime(t) =3𝔽
=DayWithinYear(t) -119𝔽 -InLeapYear(t) ifMonthFromTime(t) =4𝔽
=DayWithinYear(t) -150𝔽 -InLeapYear(t) ifMonthFromTime(t) =5𝔽
=DayWithinYear(t) -180𝔽 -InLeapYear(t) ifMonthFromTime(t) =6𝔽
=DayWithinYear(t) -211𝔽 -InLeapYear(t) ifMonthFromTime(t) =7𝔽
=DayWithinYear(t) -242𝔽 -InLeapYear(t) ifMonthFromTime(t) =8𝔽
=DayWithinYear(t) -272𝔽 -InLeapYear(t) ifMonthFromTime(t) =9𝔽
=DayWithinYear(t) -303𝔽 -InLeapYear(t) ifMonthFromTime(t) =10𝔽
=DayWithinYear(t) -333𝔽 -InLeapYear(t) ifMonthFromTime(t) =11𝔽

21.4.1.6 Week Day

The weekday for a particulartime valuet is defined as

WeekDay(t) =𝔽((Day(t) +4𝔽)modulo7)

A weekday value of+0𝔽 specifies Sunday;1𝔽 specifies Monday;2𝔽 specifies Tuesday;3𝔽 specifies Wednesday;4𝔽 specifies Thursday;5𝔽 specifies Friday; and6𝔽 specifies Saturday. Note thatWeekDay(+0𝔽) =4𝔽, corresponding to Thursday, 1 January 1970.

21.4.1.7 LocalTZA ( t, isUTC )

Theimplementation-definedabstract operation LocalTZA takes arguments t and isUTC. It returns anintegral Numberrepresenting the local time zone adjustment, or offset, in milliseconds. The local political rules for standard time and daylight saving time in effect at t should be used to determine the result in the way specified in this section.

When isUTC is true,LocalTZA( tUTC, true )should return the offset of the local time zone from UTC measured in milliseconds at time represented bytime valuetUTC. When the result is added totUTC, it should yield the corresponding Numbertlocal.

When isUTC is false,LocalTZA( tlocal, false )should return the offset of the local time zone from UTC measured in milliseconds at local time represented by Numbertlocal. When the result is subtracted fromtlocal, it should yield the correspondingtime valuetUTC.

Input t is nominally atime valuebut may be anyNumber value. This can occur when isUTC is false and tlocal represents atime valuethat is already offset outside of thetime valuerange at the range boundaries. The algorithm must not limit tlocal to thetime valuerange, so that such inputs are supported.

Whentlocalrepresents local time repeating multiple times at a negative time zone transition (e.g. when the daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or skipped local time at a positive time zone transitions (e.g. when the daylight saving time starts or the time zone offset is increased due to a time zone rule change),tlocalmust be interpreted using the time zone offset before the transition.

If an implementation does not support a conversion described above or if political rules for time t are not available within the implementation, the result must be+0𝔽.

Note

It is recommended that implementations use the time zone information of the IANA Time Zone Database https://www.iana.org/time-zones/.

1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05. LocalTZA(TimeClip(MakeDate(MakeDay(2017, 10, 5),MakeTime(1, 30, 0, 0))), false) is-4 ×msPerHour.

2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04). LocalTZA(TimeClip(MakeDate(MakeDay(2017, 2, 12),MakeTime(2, 30, 0, 0))), false) is-5 ×msPerHour.

Local time zone offset values may be positive or negative.

21.4.1.8 LocalTime ( t )

The abstract operation LocalTime takes argument t. It converts t from UTC to local time. It performs the following steps when called:

  1. Return t +LocalTZA(t,true).
Note

Two different input time valuestUTCare converted to the same local timetlocalat a negative time zone transition when there are repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).

LocalTime(UTC(tlocal))is not necessarily always equal totlocal. Correspondingly,UTC(LocalTime(tUTC))is not necessarily always equal totUTC.

21.4.1.9 UTC ( t )

The abstract operation UTC takes argument t. It converts t from local time to UTC. It performs the following steps when called:

  1. Return t -LocalTZA(t,false).
Note

UTC(LocalTime(tUTC))is not necessarily always equal totUTC. Correspondingly,LocalTime(UTC(tlocal))is not necessarily always equal totlocal.

21.4.1.10 Hours, Minutes, Second, and Milliseconds

The followingabstract operationsare useful in decomposing time values:

HourFromTime(t) =𝔽(floor((t /msPerHour))moduloHoursPerDay)
msFromTime(t) =𝔽((t)modulomsPerSecond)

where

HoursPerDay = 24
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond =1000𝔽
msPerMinute =60000𝔽 =msPerSecond×𝔽(SecondsPerMinute)
msPerHour =3600000𝔽 =msPerMinute×𝔽(MinutesPerHour)

21.4.1.11 MakeTime ( hour, min, sec, ms )

The abstract operation MakeTime takes arguments hour (a Number), min (a Number), sec (a Number), and ms (a Number). It calculates a number of milliseconds. It performs the following steps when called:

  1. If hour is not finite or min is not finite or sec is not finite or ms is not finite, returnNaN.
  2. Let h be𝔽(!ToIntegerOrInfinity(hour)).
  3. Let m be𝔽(!ToIntegerOrInfinity(min)).
  4. Let s be𝔽(!ToIntegerOrInfinity(sec)).
  5. Let milli be𝔽(!ToIntegerOrInfinity(ms)).
  6. Let t be ((h *msPerHour+ m *msPerMinute) + s *msPerSecond) + milli, performing the arithmetic according toIEEE 754-2019rules (that is, as if using the ECMAScript operators * and +).
  7. Return t.

21.4.1.12 MakeDay ( year, month, date )

The abstract operation MakeDay takes arguments year (a Number), month (a Number), and date (a Number). It calculates a number of days. It performs the following steps when called:

  1. If year is not finite or month is not finite or date is not finite, returnNaN.
  2. Let y be𝔽(!ToIntegerOrInfinity(year)).
  3. Let m be𝔽(!ToIntegerOrInfinity(month)).
  4. Let dt be𝔽(!ToIntegerOrInfinity(date)).
  5. Let ym be y +𝔽(floor((m) / 12)).
  6. If ym is not finite, returnNaN.
  7. Let mn be𝔽((m)modulo12).
  8. Find a finitetime valuet such thatYearFromTime(t) is ym andMonthFromTime(t) is mn andDateFromTime(t) is1𝔽; but if this is not possible (because some argument is out of range), returnNaN.
  9. ReturnDay(t) + dt -1𝔽.

21.4.1.13 MakeDate ( day, time )

The abstract operation MakeDate takes arguments day (a Number) and time (a Number). It calculates a number of milliseconds. It performs the following steps when called:

  1. If day is not finite or time is not finite, returnNaN.
  2. Let tv be day ×msPerDay+ time.
  3. If tv is not finite, returnNaN.
  4. Return tv.

21.4.1.14 TimeClip ( time )

The abstract operation TimeClip takes argument time (a Number). It calculates a number of milliseconds. It performs the following steps when called:

  1. If time is not finite, returnNaN.
  2. Ifabs((time)) > 8.64 × 1015, returnNaN.
  3. Return𝔽(!ToIntegerOrInfinity(time)).

21.4.1.15 Date Time String Format

ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601 calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ

Where the elements are as follows:

YYYYis the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as anexpanded yearof"+"or"-"followed by six decimal digits.
-"-"(hyphen) appears literally twice in the string.
MMis the month of the year as two decimal digits from 01 (January) to 12 (December).
DDis the day of the month as two decimal digits from 01 to 31.
T"T"appears literally in the string, to indicate the beginning of the time element.
HHis the number of complete hours that have passed since midnight as two decimal digits from 00 to 24.
:":"(colon) appears literally twice in the string.
mmis the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ssis the number of complete seconds since the start of the minute as two decimal digits from 00 to 59.
."."(dot) appears literally in the string.
sssis the number of complete milliseconds since the start of the second as three decimal digits.
Zis the UTC offset representation specified as"Z"(for UTC with no offset) or an offset of either"+"or"-"followed by a time expression HH:mm (indicating local time ahead of or behind UTC, respectively)

This format includes date-only forms:

YYYY
YYYY-MM
YYYY-MM-DD
        

It also includes “date-time” forms that consist of one of the above date-only forms immediately followed by one of the following time forms with an optional UTC offset representation appended:

THH:mm
THH:mm:ss
THH:mm:ss.sss
        

A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.

Note 1

As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. This means that the following two notations refer to exactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves it for describing time intervals and does not permit it within representations of single points in time.

Note 2

There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this reason, both ISO 8601 and this format specify numeric representations of time zone offsets.

21.4.1.15.1 Expanded Years

Coveringthe fulltime valuerange of approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1) requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year representation, but only by mutual agreement of the partners in information interchange. In the simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always prefixed with a + or - sign. The year 0 is considered positive and hence prefixed with a + sign. Strings matching theDate Time String Formatwith expanded years representing instants in time outside the range of atime valueare treated as unrecognizable byDate.parseand cause that function to returnNaNwithout falling back to implementation-specific behaviour or heuristics.

Note

Examples of date-time values with expanded years:

-271821-04-20T00:00:00Z271822 B.C.
-000001-01-01T00:00:00Z2 B.C.
+000000-01-01T00:00:00Z1 B.C.
+000001-01-01T00:00:00Z1 A.D.
+001970-01-01T00:00:00Z1970 A.D.
+002009-12-15T00:00:00Z2009 A.D.
+275760-09-13T00:00:00Z275760 A.D.

21.4.2 The Date Constructor

The Dateconstructor:

  • is %Date%.
  • is the initial value of the"Date"property of theglobal object.
  • creates and initializes a new Date object when called as aconstructor.
  • returns a String representing the current time (UTC) when called as a function rather than as aconstructor.
  • is a function whose behaviour differs based upon the number and types of its arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Date behaviour must include a super call to the Dateconstructorto create and initialize the subclass instance with a [[DateValue]] internal slot.
  • has a"length"property whose value is7𝔽.

21.4.2.1 Date ( ...values )

When the Date function is called, the following steps are taken:

  1. If NewTarget isundefined, then
    1. Let now be thetime value(UTC) identifying the current time.
    2. ReturnToDateString(now).
  2. Let numberOfArgs be the number of elements in values.
  3. If numberOfArgs = 0, then
    1. Let dv be thetime value(UTC) identifying the current time.
  4. Else if numberOfArgs = 1, then
    1. Let value be values[0].
    2. IfType(value) is Object and value has a [[DateValue]] internal slot, then
      1. Let tv be ! thisTimeValue(value).
    3. Else,
      1. Let v be ? ToPrimitive(value).
      2. IfType(v) is String, then
        1. Assert: The next step never returns anabrupt completionbecauseType(v) is String.
        2. Let tv be the result of parsing v as a date, in exactly the same manner as for the parse method (21.4.3.2).
      3. Else,
        1. Let tv be ? ToNumber(v).
    4. Let dv beTimeClip(tv).
  5. Else,
    1. Assert: numberOfArgs ≥ 2.
    2. Let y be ? ToNumber(values[0]).
    3. Let m be ? ToNumber(values[1]).
    4. If numberOfArgs > 2, let dt be ? ToNumber(values[2]); else let dt be1𝔽.
    5. If numberOfArgs > 3, let h be ? ToNumber(values[3]); else let h be+0𝔽.
    6. If numberOfArgs > 4, let min be ? ToNumber(values[4]); else let min be+0𝔽.
    7. If numberOfArgs > 5, let s be ? ToNumber(values[5]); else let s be+0𝔽.
    8. If numberOfArgs > 6, let milli be ? ToNumber(values[6]); else let milli be+0𝔽.
    9. If y isNaN, let yr beNaN.
    10. Else,
      1. Let yi be ! ToIntegerOrInfinity(y).
      2. If 0 ≤ yi ≤ 99, let yr be1900𝔽 +𝔽(yi); otherwise, let yr be y.
    11. Let finalDate beMakeDate(MakeDay(yr, m, dt),MakeTime(h, min, s, milli)).
    12. Let dv beTimeClip(UTC(finalDate)).
  6. Let O be ? OrdinaryCreateFromConstructor(NewTarget,"%Date.prototype%", « [[DateValue]] »).
  7. Set O.[[DateValue]] to dv.
  8. Return O.

21.4.3 Properties of the Date Constructor

The Dateconstructor:

21.4.3.1 Date.now ( )

The now function returns thetime valuedesignating the UTC date and time of the occurrence of the call to now.

21.4.3.2 Date.parse ( string )

The parse function applies theToStringoperator to its argument. IfToStringresults in anabrupt completiontheCompletion Recordis immediately returned. Otherwise, parse interprets the resulting String as a date and time; it returns a Number, the UTCtime valuecorresponding to the date and time. The String may be interpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function first attempts to parse the String according to the format described in Date Time String Format (21.4.1.15), including expanded years. If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats. Strings that are unrecognizable or contain out-of-bounds format element values shall cause Date.parse to returnNaN.

If the String conforms to theDate Time String Format, substitute values take the place of absent format elements. When the MM or DD elements are absent,"01"is used. When the HH, mm, or ss elements are absent,"00"is used. When the sss element is absent,"000"is used. When the UTC offset representation is absent, date-only forms are interpreted as a UTC time and date-time forms are interpreted as a local time.

If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript, then all of the following expressions should produce the same numeric value in that implementation, if all the properties referenced have their initial values:

x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())

However, the expression

Date.parse(x.toLocaleString())

is not required to produce the sameNumber valueas the preceding three expressions and, in general, the value produced by Date.parse isimplementation-definedwhen given any String value that does not conform to the Date Time String Format (21.4.1.15) and that could not be produced in that implementation by the toString or toUTCString method.

21.4.3.3 Date.prototype

The initial value of Date.prototype is theDate prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

21.4.3.4 Date.UTC ( year [ , month [ , date [ , hours [ , minutes [ , seconds [ , ms ] ] ] ] ] ] )

When the UTC function is called, the following steps are taken:

  1. Let y be ? ToNumber(year).
  2. If month is present, let m be ? ToNumber(month); else let m be+0𝔽.
  3. If date is present, let dt be ? ToNumber(date); else let dt be1𝔽.
  4. If hours is present, let h be ? ToNumber(hours); else let h be+0𝔽.
  5. If minutes is present, let min be ? ToNumber(minutes); else let min be+0𝔽.
  6. If seconds is present, let s be ? ToNumber(seconds); else let s be+0𝔽.
  7. If ms is present, let milli be ? ToNumber(ms); else let milli be+0𝔽.
  8. If y isNaN, let yr beNaN.
  9. Else,
    1. Let yi be ! ToIntegerOrInfinity(y).
    2. If 0 ≤ yi ≤ 99, let yr be1900𝔽 +𝔽(yi); otherwise, let yr be y.
  10. ReturnTimeClip(MakeDate(MakeDay(yr, m, dt),MakeTime(h, min, s, milli))).

The"length"property of the UTC function is7𝔽.

Note

The UTC function differs from the Dateconstructorin two ways: it returns atime valueas a Number, rather than creating a Date object, and it interprets the arguments in UTC rather than as local time.

21.4.4 Properties of the Date Prototype Object

The Date prototype object:

  • is %Date.prototype%.
  • is itself anordinary object.
  • is not a Date instance and does not have a [[DateValue]] internal slot.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic and thethisvalue passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to atime value.

The abstract operation thisTimeValue takes argument value. It performs the following steps when called:

  1. IfType(value) is Object and value has a [[DateValue]] internal slot, then
    1. Return value.[[DateValue]].
  2. Throw aTypeErrorexception.

In following descriptions of functions that are properties of the Date prototype object, the phrase “this Date object” refers to the object that is thethisvalue for the invocation of the function. If the Type of thethisvalue is not Object, aTypeErrorexception is thrown. The phrase “this time value” within the specification of a method refers to the result returned by calling the abstract operationthisTimeValuewith thethisvalue of the method invocation passed as the argument.

21.4.4.1 Date.prototype.constructor

The initial value of Date.prototype.constructor is%Date%.

21.4.4.2 Date.prototype.getDate ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnDateFromTime(LocalTime(t)).

21.4.4.3 Date.prototype.getDay ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnWeekDay(LocalTime(t)).

21.4.4.4 Date.prototype.getFullYear ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnYearFromTime(LocalTime(t)).

21.4.4.5 Date.prototype.getHours ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnHourFromTime(LocalTime(t)).

21.4.4.6 Date.prototype.getMilliseconds ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnmsFromTime(LocalTime(t)).

21.4.4.7 Date.prototype.getMinutes ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnMinFromTime(LocalTime(t)).

21.4.4.8 Date.prototype.getMonth ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnMonthFromTime(LocalTime(t)).

21.4.4.9 Date.prototype.getSeconds ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnSecFromTime(LocalTime(t)).

21.4.4.10 Date.prototype.getTime ( )

The following steps are performed:

  1. Return ? thisTimeValue(thisvalue).

21.4.4.11 Date.prototype.getTimezoneOffset ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. Return (t -LocalTime(t)) /msPerMinute.

21.4.4.12 Date.prototype.getUTCDate ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnDateFromTime(t).

21.4.4.13 Date.prototype.getUTCDay ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnWeekDay(t).

21.4.4.14 Date.prototype.getUTCFullYear ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnYearFromTime(t).

21.4.4.15 Date.prototype.getUTCHours ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnHourFromTime(t).

21.4.4.16 Date.prototype.getUTCMilliseconds ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnmsFromTime(t).

21.4.4.17 Date.prototype.getUTCMinutes ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnMinFromTime(t).

21.4.4.18 Date.prototype.getUTCMonth ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnMonthFromTime(t).

21.4.4.19 Date.prototype.getUTCSeconds ( )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnSecFromTime(t).

21.4.4.20 Date.prototype.setDate ( date )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Let dt be ? ToNumber(date).
  3. Let newDate beMakeDate(MakeDay(YearFromTime(t),MonthFromTime(t), dt),TimeWithinDay(t)).
  4. Let u beTimeClip(UTC(newDate)).
  5. Set the [[DateValue]] internal slot ofthis Date objectto u.
  6. Return u.

21.4.4.21 Date.prototype.setFullYear ( year [ , month [ , date ] ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, set t to+0𝔽; otherwise, set t toLocalTime(t).
  3. Let y be ? ToNumber(year).
  4. If month is not present, let m beMonthFromTime(t); otherwise, let m be ? ToNumber(month).
  5. If date is not present, let dt beDateFromTime(t); otherwise, let dt be ? ToNumber(date).
  6. Let newDate beMakeDate(MakeDay(y, m, dt),TimeWithinDay(t)).
  7. Let u beTimeClip(UTC(newDate)).
  8. Set the [[DateValue]] internal slot ofthis Date objectto u.
  9. Return u.

The"length"property of the setFullYear method is3𝔽.

Note

If month is not present, this method behaves as if month was present with the value getMonth(). If date is not present, it behaves as if date was present with the value getDate().

21.4.4.22 Date.prototype.setHours ( hour [ , min [ , sec [ , ms ] ] ] )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Let h be ? ToNumber(hour).
  3. If min is not present, let m beMinFromTime(t); otherwise, let m be ? ToNumber(min).
  4. If sec is not present, let s beSecFromTime(t); otherwise, let s be ? ToNumber(sec).
  5. If ms is not present, let milli bemsFromTime(t); otherwise, let milli be ? ToNumber(ms).
  6. Let date beMakeDate(Day(t),MakeTime(h, m, s, milli)).
  7. Let u beTimeClip(UTC(date)).
  8. Set the [[DateValue]] internal slot ofthis Date objectto u.
  9. Return u.

The"length"property of the setHours method is4𝔽.

Note

If min is not present, this method behaves as if min was present with the value getMinutes(). If sec is not present, it behaves as if sec was present with the value getSeconds(). If ms is not present, it behaves as if ms was present with the value getMilliseconds().

21.4.4.23 Date.prototype.setMilliseconds ( ms )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Set ms to ? ToNumber(ms).
  3. Let time beMakeTime(HourFromTime(t),MinFromTime(t),SecFromTime(t), ms).
  4. Let u beTimeClip(UTC(MakeDate(Day(t), time))).
  5. Set the [[DateValue]] internal slot ofthis Date objectto u.
  6. Return u.

21.4.4.24 Date.prototype.setMinutes ( min [ , sec [ , ms ] ] )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Let m be ? ToNumber(min).
  3. If sec is not present, let s beSecFromTime(t); otherwise, let s be ? ToNumber(sec).
  4. If ms is not present, let milli bemsFromTime(t); otherwise, let milli be ? ToNumber(ms).
  5. Let date beMakeDate(Day(t),MakeTime(HourFromTime(t), m, s, milli)).
  6. Let u beTimeClip(UTC(date)).
  7. Set the [[DateValue]] internal slot ofthis Date objectto u.
  8. Return u.

The"length"property of the setMinutes method is3𝔽.

Note

If sec is not present, this method behaves as if sec was present with the value getSeconds(). If ms is not present, this behaves as if ms was present with the value getMilliseconds().

21.4.4.25 Date.prototype.setMonth ( month [ , date ] )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Let m be ? ToNumber(month).
  3. If date is not present, let dt beDateFromTime(t); otherwise, let dt be ? ToNumber(date).
  4. Let newDate beMakeDate(MakeDay(YearFromTime(t), m, dt),TimeWithinDay(t)).
  5. Let u beTimeClip(UTC(newDate)).
  6. Set the [[DateValue]] internal slot ofthis Date objectto u.
  7. Return u.

The"length"property of the setMonth method is2𝔽.

Note

If date is not present, this method behaves as if date was present with the value getDate().

21.4.4.26 Date.prototype.setSeconds ( sec [ , ms ] )

The following steps are performed:

  1. Let t beLocalTime(?thisTimeValue(thisvalue)).
  2. Let s be ? ToNumber(sec).
  3. If ms is not present, let milli bemsFromTime(t); otherwise, let milli be ? ToNumber(ms).
  4. Let date beMakeDate(Day(t),MakeTime(HourFromTime(t),MinFromTime(t), s, milli)).
  5. Let u beTimeClip(UTC(date)).
  6. Set the [[DateValue]] internal slot ofthis Date objectto u.
  7. Return u.

The"length"property of the setSeconds method is2𝔽.

Note

If ms is not present, this method behaves as if ms was present with the value getMilliseconds().

21.4.4.27 Date.prototype.setTime ( time )

The following steps are performed:

  1. Perform ? thisTimeValue(thisvalue).
  2. Let t be ? ToNumber(time).
  3. Let v beTimeClip(t).
  4. Set the [[DateValue]] internal slot ofthis Date objectto v.
  5. Return v.

21.4.4.28 Date.prototype.setUTCDate ( date )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let dt be ? ToNumber(date).
  3. Let newDate beMakeDate(MakeDay(YearFromTime(t),MonthFromTime(t), dt),TimeWithinDay(t)).
  4. Let v beTimeClip(newDate).
  5. Set the [[DateValue]] internal slot ofthis Date objectto v.
  6. Return v.

21.4.4.29 Date.prototype.setUTCFullYear ( year [ , month [ , date ] ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, set t to+0𝔽.
  3. Let y be ? ToNumber(year).
  4. If month is not present, let m beMonthFromTime(t); otherwise, let m be ? ToNumber(month).
  5. If date is not present, let dt beDateFromTime(t); otherwise, let dt be ? ToNumber(date).
  6. Let newDate beMakeDate(MakeDay(y, m, dt),TimeWithinDay(t)).
  7. Let v beTimeClip(newDate).
  8. Set the [[DateValue]] internal slot ofthis Date objectto v.
  9. Return v.

The"length"property of the setUTCFullYear method is3𝔽.

Note

If month is not present, this method behaves as if month was present with the value getUTCMonth(). If date is not present, it behaves as if date was present with the value getUTCDate().

21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [ , sec [ , ms ] ] ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let h be ? ToNumber(hour).
  3. If min is not present, let m beMinFromTime(t); otherwise, let m be ? ToNumber(min).
  4. If sec is not present, let s beSecFromTime(t); otherwise, let s be ? ToNumber(sec).
  5. If ms is not present, let milli bemsFromTime(t); otherwise, let milli be ? ToNumber(ms).
  6. Let newDate beMakeDate(Day(t),MakeTime(h, m, s, milli)).
  7. Let v beTimeClip(newDate).
  8. Set the [[DateValue]] internal slot ofthis Date objectto v.
  9. Return v.

The"length"property of the setUTCHours method is4𝔽.

Note

If min is not present, this method behaves as if min was present with the value getUTCMinutes(). If sec is not present, it behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it behaves as if ms was present with the value getUTCMilliseconds().

21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let milli be ? ToNumber(ms).
  3. Let time beMakeTime(HourFromTime(t),MinFromTime(t),SecFromTime(t), milli).
  4. Let v beTimeClip(MakeDate(Day(t), time)).
  5. Set the [[DateValue]] internal slot ofthis Date objectto v.
  6. Return v.

21.4.4.32 Date.prototype.setUTCMinutes ( min [ , sec [ , ms ] ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let m be ? ToNumber(min).
  3. If sec is not present, let s beSecFromTime(t).
  4. Else,
    1. Let s be ? ToNumber(sec).
  5. If ms is not present, let milli bemsFromTime(t).
  6. Else,
    1. Let milli be ? ToNumber(ms).
  7. Let date beMakeDate(Day(t),MakeTime(HourFromTime(t), m, s, milli)).
  8. Let v beTimeClip(date).
  9. Set the [[DateValue]] internal slot ofthis Date objectto v.
  10. Return v.

The"length"property of the setUTCMinutes method is3𝔽.

Note

If sec is not present, this method behaves as if sec was present with the value getUTCSeconds(). If ms is not present, it function behaves as if ms was present with the value return by getUTCMilliseconds().

21.4.4.33 Date.prototype.setUTCMonth ( month [ , date ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let m be ? ToNumber(month).
  3. If date is not present, let dt beDateFromTime(t).
  4. Else,
    1. Let dt be ? ToNumber(date).
  5. Let newDate beMakeDate(MakeDay(YearFromTime(t), m, dt),TimeWithinDay(t)).
  6. Let v beTimeClip(newDate).
  7. Set the [[DateValue]] internal slot ofthis Date objectto v.
  8. Return v.

The"length"property of the setUTCMonth method is2𝔽.

Note

If date is not present, this method behaves as if date was present with the value getUTCDate().

21.4.4.34 Date.prototype.setUTCSeconds ( sec [ , ms ] )

The following steps are performed:

  1. Let t be ? thisTimeValue(thisvalue).
  2. Let s be ? ToNumber(sec).
  3. If ms is not present, let milli bemsFromTime(t).
  4. Else,
    1. Let milli be ? ToNumber(ms).
  5. Let date beMakeDate(Day(t),MakeTime(HourFromTime(t),MinFromTime(t), s, milli)).
  6. Let v beTimeClip(date).
  7. Set the [[DateValue]] internal slot ofthis Date objectto v.
  8. Return v.

The"length"property of the setUTCSeconds method is2𝔽.

Note

If ms is not present, this method behaves as if ms was present with the value getUTCMilliseconds().

21.4.4.35 Date.prototype.toDateString ( )

The following steps are performed:

  1. Let O bethis Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv isNaN, return"Invalid Date".
  4. Let t beLocalTime(tv).
  5. ReturnDateString(t).

21.4.4.36 Date.prototype.toISOString ( )

Ifthis time valueis not a finite Number or if it corresponds with a year that cannot be represented in theDate Time String Format, this function throws aRangeErrorexception. Otherwise, it returns a String representation ofthis time valuein that format on the UTC time scale, including all format elements and the UTC offset representation"Z".

21.4.4.37 Date.prototype.toJSON ( key )

This function provides a String representation of a Date object for use by JSON.stringify (25.5.2).

When the toJSON method is called with argument key, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let tv be ? ToPrimitive(O,number).
  3. IfType(tv) is Number and tv is not finite, returnnull.
  4. Return ? Invoke(O,"toISOString").
Note 1

The argument is ignored.

Note 2

The toJSON function is intentionally generic; it does not require that itsthisvalue be a Date object. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have a toISOString method.

21.4.4.38 Date.prototype.toLocaleDateString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleDateString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleDateString method is used.

This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.39 Date.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.40 Date.prototype.toLocaleTimeString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Date.prototype.toLocaleTimeString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleTimeString method is used.

This function returns a String value. The contents of the String areimplementation-defined, but are intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable form that corresponds to the conventions of thehost environment's current locale.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

21.4.4.41 Date.prototype.toString ( )

The following steps are performed:

  1. Let tv be ? thisTimeValue(thisvalue).
  2. ReturnToDateString(tv).
Note 1

For any Date object d such that d.[[DateValue]] is evenly divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See21.4.3.2.

Note 2

The toString function is not generic; it throws aTypeErrorexception if itsthisvalue is not a Date object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

21.4.4.41.1 TimeString ( tv )

The abstract operation TimeString takes argument tv. It performs the following steps when called:

  1. Assert:Type(tv) is Number.
  2. Assert: tv is notNaN.
  3. Let hour be the String representation ofHourFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  4. Let minute be the String representation ofMinFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  5. Let second be the String representation ofSecFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  6. Return thestring-concatenationof hour,":", minute,":", second, the code unit 0x0020 (SPACE), and"GMT".

21.4.4.41.2 DateString ( tv )

The abstract operation DateString takes argument tv. It performs the following steps when called:

  1. Assert:Type(tv) is Number.
  2. Assert: tv is notNaN.
  3. Let weekday be the Name of the entry inTable 55with the NumberWeekDay(tv).
  4. Let month be the Name of the entry inTable 56with the NumberMonthFromTime(tv).
  5. Let day be the String representation ofDateFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  6. Let yv beYearFromTime(tv).
  7. If yv+0𝔽, let yearSign be the empty String; otherwise, let yearSign be"-".
  8. Let year be the String representation ofabs((yv)), formatted as a decimal number.
  9. Let paddedYear be ! StringPad(year,4𝔽,"0",start).
  10. Return thestring-concatenationof weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
Table 55: Names of days of the week
NumberName
+0𝔽"Sun"
1𝔽"Mon"
2𝔽"Tue"
3𝔽"Wed"
4𝔽"Thu"
5𝔽"Fri"
6𝔽"Sat"
Table 56: Names of months of the year
NumberName
+0𝔽"Jan"
1𝔽"Feb"
2𝔽"Mar"
3𝔽"Apr"
4𝔽"May"
5𝔽"Jun"
6𝔽"Jul"
7𝔽"Aug"
8𝔽"Sep"
9𝔽"Oct"
10𝔽"Nov"
11𝔽"Dec"

21.4.4.41.3 TimeZoneString ( tv )

The abstract operation TimeZoneString takes argument tv. It performs the following steps when called:

  1. Assert:Type(tv) is Number.
  2. Assert: tv is notNaN.
  3. Let offset beLocalTZA(tv,true).
  4. If offset+0𝔽, then
    1. Let offsetSign be"+".
    2. Let absOffset be offset.
  5. Else,
    1. Let offsetSign be"-".
    2. Let absOffset be -offset.
  6. Let offsetMin be the String representation ofMinFromTime(absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  7. Let offsetHour be the String representation ofHourFromTime(absOffset), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  8. Let tzName be animplementation-definedstring that is either the empty String or thestring-concatenationof the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), animplementation-definedtimezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
  9. Return thestring-concatenationof offsetSign, offsetHour, offsetMin, and tzName.

21.4.4.41.4 ToDateString ( tv )

The abstract operation ToDateString takes argument tv. It performs the following steps when called:

  1. Assert:Type(tv) is Number.
  2. If tv isNaN, return"Invalid Date".
  3. Let t beLocalTime(tv).
  4. Return thestring-concatenationofDateString(t), the code unit 0x0020 (SPACE),TimeString(t), andTimeZoneString(tv).

21.4.4.42 Date.prototype.toTimeString ( )

The following steps are performed:

  1. Let O bethis Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv isNaN, return"Invalid Date".
  4. Let t beLocalTime(tv).
  5. Return thestring-concatenationofTimeString(t) andTimeZoneString(tv).

21.4.4.43 Date.prototype.toUTCString ( )

The toUTCString method returns a String value representing the instance in time corresponding tothis time value. The format of the String is based upon "HTTP-date" from RFC 7231, generalized to support the full range of times supported by ECMAScript Date objects. It performs the following steps when called:

  1. Let O bethis Date object.
  2. Let tv be ? thisTimeValue(O).
  3. If tv isNaN, return"Invalid Date".
  4. Let weekday be the Name of the entry inTable 55with the NumberWeekDay(tv).
  5. Let month be the Name of the entry inTable 56with the NumberMonthFromTime(tv).
  6. Let day be the String representation ofDateFromTime(tv), formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
  7. Let yv beYearFromTime(tv).
  8. If yv+0𝔽, let yearSign be the empty String; otherwise, let yearSign be"-".
  9. Let year be the String representation ofabs((yv)), formatted as a decimal number.
  10. Let paddedYear be ! StringPad(year,4𝔽,"0",start).
  11. Return thestring-concatenationof weekday,",", the code unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE), andTimeString(tv).

21.4.4.44 Date.prototype.valueOf ( )

The following steps are performed:

  1. Return ? thisTimeValue(thisvalue).

21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )

This function is called by ECMAScript language operators to convert a Date object to a primitive value. The allowed values for hint are"default","number", and"string". Date objects, are unique among built-in ECMAScript object in that they treat"default"as being equivalent to"string", All other built-in ECMAScript objects treat"default"as being equivalent to"number".

When the @@toPrimitive method is called with argument hint, the following steps are taken:

  1. Let O be thethisvalue.
  2. IfType(O) is not Object, throw aTypeErrorexception.
  3. If hint is"string"or"default", then
    1. Let tryFirst bestring.
  4. Else if hint is"number", then
    1. Let tryFirst benumber.
  5. Else, throw aTypeErrorexception.
  6. Return ? OrdinaryToPrimitive(O, tryFirst).

The value of the"name"property of this function is"[Symbol.toPrimitive]".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

21.4.5 Properties of Date Instances

Date instances are ordinary objects that inherit properties from theDate prototype object. Date instances also have a [[DateValue]] internal slot. The [[DateValue]] internal slot is thetime valuerepresented bythis Date object.

22 Text Processing

22.1 String Objects

22.1.1 The String Constructor

The Stringconstructor:

  • is %String%.
  • is the initial value of the"String"property of theglobal object.
  • creates and initializes a new String object when called as aconstructor.
  • performs a type conversion when called as a function rather than as aconstructor.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified String behaviour must include a super call to the Stringconstructorto create and initialize the subclass instance with a [[StringData]] internal slot.

22.1.1.1 String ( value )

When String is called with argument value, the following steps are taken:

  1. If value is not present, let s be the empty String.
  2. Else,
    1. If NewTarget isundefinedandType(value) is Symbol, returnSymbolDescriptiveString(value).
    2. Let s be ? ToString(value).
  3. If NewTarget isundefined, return s.
  4. Return ! StringCreate(s, ? GetPrototypeFromConstructor(NewTarget,"%String.prototype%")).

22.1.2 Properties of the String Constructor

The Stringconstructor:

22.1.2.1 String.fromCharCode ( ...codeUnits )

The String.fromCharCode function may be called with any number of arguments which form the rest parameter codeUnits. The following steps are taken:

  1. Let length be the number of elements in codeUnits.
  2. Let elements be a new emptyList.
  3. For each element next of codeUnits, do
    1. Let nextCU be(?ToUint16(next)).
    2. Append nextCU to the end of elements.
  4. Return the String value whose code units are the elements in theListelements. If codeUnits is empty, the empty String is returned.

The"length"property of the fromCharCode function is1𝔽.

22.1.2.2 String.fromCodePoint ( ...codePoints )

The String.fromCodePoint function may be called with any number of arguments which form the rest parameter codePoints. The following steps are taken:

  1. Let result be the empty String.
  2. For each element next of codePoints, do
    1. Let nextCP be ? ToNumber(next).
    2. If ! IsIntegralNumber(nextCP) isfalse, throw aRangeErrorexception.
    3. If(nextCP) < 0 or(nextCP) > 0x10FFFF, throw aRangeErrorexception.
    4. Set result to thestring-concatenationof result and ! UTF16EncodeCodePoint((nextCP)).
  3. Assert: If codePoints is empty, then result is the empty String.
  4. Return result.

The"length"property of the fromCodePoint function is1𝔽.

22.1.2.3 String.prototype

The initial value of String.prototype is theString prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

22.1.2.4 String.raw ( template, ...substitutions )

The String.raw function may be called with a variable number of arguments. The first argument is template and the remainder of the arguments form theListsubstitutions. The following steps are taken:

  1. Let numberOfSubstitutions be the number of elements in substitutions.
  2. Let cooked be ? ToObject(template).
  3. Let raw be ? ToObject(?Get(cooked,"raw")).
  4. Let literalSegments be ? LengthOfArrayLike(raw).
  5. If literalSegments ≤ 0, return the empty String.
  6. Let stringElements be a new emptyList.
  7. Let nextIndex be 0.
  8. Repeat,
    1. Let nextKey be ! ToString(𝔽(nextIndex)).
    2. Let nextSeg be ? ToString(?Get(raw, nextKey)).
    3. Append the code unit elements of nextSeg to the end of stringElements.
    4. If nextIndex + 1 = literalSegments, then
      1. Return the String value whose code units are the elements in theListstringElements. If stringElements has no elements, the empty String is returned.
    5. If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
    6. Else, let next be the empty String.
    7. Let nextSub be ? ToString(next).
    8. Append the code unit elements of nextSub to the end of stringElements.
    9. Set nextIndex to nextIndex + 1.
Note

The raw function is intended for use as a tag function of a Tagged Template (13.3.11). When called as such, the first argument will be a well formed template object and the rest parameter will contain the substitution values.

22.1.3 Properties of the String Prototype Object

The String prototype object:

  • is %String.prototype%.
  • is aString exotic objectand has the internal methods specified for such objects.
  • has a [[StringData]] internal slot whose value is the empty String.
  • has a"length"property whose initial value is+0𝔽 and whose attributes are { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.

Unless explicitly stated otherwise, the methods of the String prototype object defined below are not generic and thethisvalue passed to them must be either a String value or an object that has a [[StringData]] internal slot that has been initialized to a String value.

The abstract operation thisStringValue takes argument value. It performs the following steps when called:

  1. IfType(value) is String, return value.
  2. IfType(value) is Object and value has a [[StringData]] internal slot, then
    1. Let s be value.[[StringData]].
    2. Assert:Type(s) is String.
    3. Return s.
  3. Throw aTypeErrorexception.

22.1.3.1 String.prototype.charAt ( pos )

Note 1

Returns a single element String containing the code unit at index pos within the String value resulting from converting this object to a String. If there is no element at that index, the result is the empty String. The result is a String value, not a String object.

If pos is anintegral Number, then the result of x.charAt(pos) is equivalent to the result of x.substring(pos, pos + 1).

When the charAt method is called with one argument pos, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or positionsize, return the empty String.
  6. Return the String value of length 1, containing one code unit from S, namely the code unit at index position.
Note 2

The charAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.2 String.prototype.charCodeAt ( pos )

Note 1

Returns a Number (a non-negativeintegral Numberless than 216) that is the numeric value of the code unit at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result isNaN.

When the charCodeAt method is called with one argument pos, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or positionsize, returnNaN.
  6. Return theNumber valuefor the numeric value of the code unit at index position within the String S.
Note 2

The charCodeAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.3 String.prototype.codePointAt ( pos )

Note 1

Returns a non-negativeintegral Numberless than or equal to0x10FFFF𝔽 that is the code point value of the UTF-16 encoded code point (6.1.4) starting at the string element at index pos within the String resulting from converting this object to a String. If there is no element at that index, the result isundefined. If a valid UTF-16surrogate pairdoes not begin at pos, the result is the code unit at pos.

When the codePointAt method is called with one argument pos, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let position be ? ToIntegerOrInfinity(pos).
  4. Let size be the length of S.
  5. If position < 0 or positionsize, returnundefined.
  6. Let cp be ! CodePointAt(S, position).
  7. Return𝔽(cp.[[CodePoint]]).
Note 2

The codePointAt function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.4 String.prototype.concat ( ...args )

Note 1

When the concat method is called it returns the String value consisting of the code units of thethisvalue (converted to a String) followed by the code units of each of the arguments converted to a String. The result is a String value, not a String object.

When the concat method is called with zero or more arguments, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let R be S.
  4. For each element next of args, do
    1. Let nextString be ? ToString(next).
    2. Set R to thestring-concatenationof R and nextString.
  5. Return R.

The"length"property of the concat method is1𝔽.

Note 2

The concat function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.5 String.prototype.constructor

The initial value of String.prototype.constructor is%String%.

22.1.3.6 String.prototype.endsWith ( searchString [ , endPosition ] )

The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp istrue, throw aTypeErrorexception.
  5. Let searchStr be ? ToString(searchString).
  6. Let len be the length of S.
  7. If endPosition isundefined, let pos be len; else let pos be ? ToIntegerOrInfinity(endPosition).
  8. Let end be the result ofclampingpos between 0 and len.
  9. Let searchLength be the length of searchStr.
  10. If searchLength = 0, returntrue.
  11. Let start be end - searchLength.
  12. If start < 0, returnfalse.
  13. Let substring be thesubstringof S from start to end.
  14. Return ! SameValueNonNumeric(substring, searchStr).
Note 1

Returnstrueif the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at endPosition - length(this). Otherwise returnsfalse.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The endsWith function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.7 String.prototype.includes ( searchString [ , position ] )

The includes method takes two arguments, searchString and position, and performs the following steps:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp istrue, throw aTypeErrorexception.
  5. Let searchStr be ? ToString(searchString).
  6. Let pos be ? ToIntegerOrInfinity(position).
  7. Assert: If position isundefined, then pos is 0.
  8. Let len be the length of S.
  9. Let start be the result ofclampingpos between 0 and len.
  10. Let index be ! StringIndexOf(S, searchStr, start).
  11. If index is not -1, returntrue.
  12. Returnfalse.
Note 1

If searchString appears as asubstringof the result of converting this object to a String, at one or more indices that are greater than or equal to position, returntrue; otherwise, returnsfalse. If position isundefined, 0 is assumed, so as to search all of the String.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The includes function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.8 String.prototype.indexOf ( searchString [ , position ] )

Note 1

If searchString appears as asubstringof the result of converting this object to a String, at one or more indices that are greater than or equal to position, then the smallest such index is returned; otherwise,-1𝔽 is returned. If position isundefined,+0𝔽 is assumed, so as to search all of the String.

The indexOf method takes two arguments, searchString and position, and performs the following steps:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let searchStr be ? ToString(searchString).
  4. Let pos be ? ToIntegerOrInfinity(position).
  5. Assert: If position isundefined, then pos is 0.
  6. Let len be the length of S.
  7. Let start be the result ofclampingpos between 0 and len.
  8. Return𝔽(!StringIndexOf(S, searchStr, start)).
Note 2

The indexOf function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.9 String.prototype.lastIndexOf ( searchString [ , position ] )

Note 1

If searchString appears as asubstringof the result of converting this object to a String at one or more indices that are smaller than or equal to position, then the greatest such index is returned; otherwise,-1𝔽 is returned. If position isundefined, the length of the String value is assumed, so as to search all of the String.

The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let searchStr be ? ToString(searchString).
  4. Let numPos be ? ToNumber(position).
  5. Assert: If position isundefined, then numPos isNaN.
  6. If numPos isNaN, let pos be +∞; otherwise, let pos be ! ToIntegerOrInfinity(numPos).
  7. Let len be the length of S.
  8. Let start be the result ofclampingpos between 0 and len.
  9. Let searchLen be the length of searchStr.
  10. Let k be the largest possible non-negativeintegernot larger than start such that k + searchLenlen, and for all non-negative integers j such that j < searchLen, the code unit at index k + j within S is the same as the code unit at index j within searchStr; but if there is no suchinteger, let k be -1.
  11. Return𝔽(k).
Note 2

The lastIndexOf function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.10 String.prototype.localeCompare ( that [ , reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the localeCompare method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the localeCompare method is used.

When the localeCompare method is called with argument that, it returns a Number other thanNaNthat represents the result of a locale-sensitive String comparison of thethisvalue (converted to a String) with that (converted to a String). The two Strings are S and That. The two Strings are compared in animplementation-definedfashion. The result is intended to order String values in the sort order specified by ahostdefault locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, the Strings are equal, or S comes after That in the sort order, respectively.

Before performing the comparisons, the following steps are performed to prepare the Strings:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let That be ? ToString(that).

The meaning of the optional second and third parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not assign any other interpretation to those parameter positions.

The localeCompare method, if considered as a function of two argumentsthisand that, is a consistent comparison function (as defined in23.1.3.27) on the set of all Strings.

The actual return values areimplementation-definedto permit implementers to encode additional information in the value, but the function is required to define a total ordering on all Strings. This function must treat Strings that are canonically equivalent according to the Unicode standard as identical and must return 0 when comparing Strings that are considered canonically equivalent.

Note 1

The localeCompare method itself is not directly suitable as an argument to Array.prototype.sort because the latter requires a function of two arguments.

Note 2

This function is intended to rely on whatever language-sensitive comparison functionality is available to the ECMAScript environment from thehost environment, and to compare according to the rules of thehost environment's current locale. However, regardless of thehostprovided comparison capabilities, this function must treat Strings that are canonically equivalent according to the Unicode standard as identical. It is recommended that this function should not honour Unicode compatibility equivalences or decompositions. For a definition and discussion of canonical equivalence see the Unicode Standard, chapters 2 and 3, as well as Unicode Standard Annex #15, Unicode Normalization Forms (https://unicode.org/reports/tr15/) and Unicode Technical Note #5, Canonical Equivalence in Applications (https://www.unicode.org/notes/tn5/). Also see Unicode Technical Standard #10, Unicode Collation Algorithm (https://unicode.org/reports/tr10/).

Note 3

The localeCompare function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.11 String.prototype.match ( regexp )

When the match method is called with argument regexp, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If regexp is neitherundefinednornull, then
    1. Let matcher be ? GetMethod(regexp,@@match).
    2. If matcher is notundefined, then
      1. Return ? Call(matcher, regexp, « O »).
  3. Let S be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp,undefined).
  5. Return ? Invoke(rx,@@match, « S »).
Note

The match function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.12 String.prototype.matchAll ( regexp )

Performs a regular expression match of the String representing thethisvalue against regexp and returns an iterator. Each iteration result's value is an Array object containing the results of the match, ornullif the String did not match.

When the matchAll method is called, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If regexp is neitherundefinednornull, then
    1. Let isRegExp be ? IsRegExp(regexp).
    2. If isRegExp istrue, then
      1. Let flags be ? Get(regexp,"flags").
      2. Perform ? RequireObjectCoercible(flags).
      3. If ? ToString(flags) does not contain"g", throw aTypeErrorexception.
    3. Let matcher be ? GetMethod(regexp,@@matchAll).
    4. If matcher is notundefined, then
      1. Return ? Call(matcher, regexp, « O »).
  3. Let S be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp,"g").
  5. Return ? Invoke(rx,@@matchAll, « S »).
Note 1
The matchAll function is intentionally generic, it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.
Note 2
Similarly to String.prototype.split, String.prototype.matchAll is designed to typically act without mutating its inputs.

22.1.3.13 String.prototype.normalize ( [ form ] )

When the normalize method is called with one argument form, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. If form isundefined, let f be"NFC".
  4. Else, let f be ? ToString(form).
  5. If f is not one of"NFC","NFD","NFKC", or"NFKD", throw aRangeErrorexception.
  6. Let ns be the String value that is the result of normalizing S into the normalization form named by f as specified in https://unicode.org/reports/tr15/.
  7. Return ns.
Note

The normalize function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.14 String.prototype.padEnd ( maxLength [ , fillString ] )

When the padEnd method is called, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Return ? StringPad(O, maxLength, fillString,end).

22.1.3.15 String.prototype.padStart ( maxLength [ , fillString ] )

When the padStart method is called, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Return ? StringPad(O, maxLength, fillString,start).

22.1.3.15.1 StringPad ( O, maxLength, fillString, placement )

The abstract operation StringPad takes arguments O, maxLength, fillString, and placement. It performs the following steps when called:

  1. Assert: placement isstartorend.
  2. Let S be ? ToString(O).
  3. Let intMaxLength be(?ToLength(maxLength)).
  4. Let stringLength be the length of S.
  5. If intMaxLengthstringLength, return S.
  6. If fillString isundefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
  7. Else, let filler be ? ToString(fillString).
  8. If filler is the empty String, return S.
  9. Let fillLen be intMaxLength - stringLength.
  10. Let truncatedStringFiller be the String value consisting of repeated concatenations of filler truncated to length fillLen.
  11. If placement isstart, return thestring-concatenationof truncatedStringFiller and S.
  12. Else, return thestring-concatenationof S and truncatedStringFiller.
Note 1

The argument maxLength will be clamped such that it can be no smaller than the length of S.

Note 2

The argument fillString defaults to" "(the String value consisting of the code unit 0x0020 SPACE).

22.1.3.16 String.prototype.repeat ( count )

The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let n be ? ToIntegerOrInfinity(count).
  4. If n < 0 or n is +∞, throw aRangeErrorexception.
  5. If n is 0, return the empty String.
  6. Return the String value that is made from n copies of S appended together.
Note 1

This method creates the String value consisting of the code units of thethisvalue (converted to String) repeated count times.

Note 2

The repeat function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.17 String.prototype.replace ( searchValue, replaceValue )

When the replace method is called with arguments searchValue and replaceValue, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If searchValue is neitherundefinednornull, then
    1. Let replacer be ? GetMethod(searchValue,@@replace).
    2. If replacer is notundefined, then
      1. Return ? Call(replacer, searchValue, « O, replaceValue »).
  3. Let string be ? ToString(O).
  4. Let searchString be ? ToString(searchValue).
  5. Let functionalReplace beIsCallable(replaceValue).
  6. If functionalReplace isfalse, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let searchLength be the length of searchString.
  8. Let position be ! StringIndexOf(string, searchString, 0).
  9. If position is -1, return string.
  10. Let preserved be thesubstringof string from 0 to position.
  11. If functionalReplace istrue, then
    1. Let replacement be ? ToString(?Call(replaceValue,undefined, « searchString,𝔽(position), string »)).
  12. Else,
    1. Assert:Type(replaceValue) is String.
    2. Let captures be a new emptyList.
    3. Let replacement be ! GetSubstitution(searchString, string, position, captures,undefined, replaceValue).
  13. Return thestring-concatenationof preserved, replacement, and thesubstringof string from position + searchLength.
Note

The replace function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.17.1 GetSubstitution ( matched, str, position, captures, namedCaptures, replacement )

The abstract operation GetSubstitution takes arguments matched, str, position (a non-negativeinteger), captures, namedCaptures, and replacement. It performs the following steps when called:

  1. Assert:Type(matched) is String.
  2. Let matchLength be the number of code units in matched.
  3. Assert:Type(str) is String.
  4. Let stringLength be the number of code units in str.
  5. Assert: positionstringLength.
  6. Assert: captures is a possibly emptyListof Strings.
  7. Assert:Type(replacement) is String.
  8. Let tailPos be position + matchLength.
  9. Let m be the number of elements in captures.
  10. Let result be the String value derived from replacement by copying code unit elements from replacement to result while performing replacements as specified inTable 57. These $ replacements are done left-to-right, and, once such a replacement is performed, the new replacement text is not subject to further replacements.
  11. Return result.
Table 57: Replacement Text Symbol Substitutions
Code unitsUnicode CharactersReplacement text
0x0024, 0x0024$$$
0x0024, 0x0026$&matched
0x0024, 0x0060$`The replacement is thesubstringof str from 0 to position.
0x0024, 0x0027$'If tailPosstringLength, the replacement is the empty String. Otherwise the replacement is thesubstringof str from tailPos.
0x0024, N
Where
0x0031 ≤ N ≤ 0x0039
$n where
n is one of 1 2 3 4 5 6 7 8 9 and $n is not followed by a decimal digit
The nth element of captures, where n is a single digit in the range 1 to 9. If nm and the nth element of captures isundefined, use the empty String instead. If n > m, no replacement is done.
0x0024, N, N
Where
0x0030 ≤ N ≤ 0x0039
$nn where
n is one of 0 1 2 3 4 5 6 7 8 9
The nnth element of captures, where nn is a two-digit decimal number in the range 01 to 99. If nnm and the nnth element of captures isundefined, use the empty String instead. If nn is 00 or nn > m, no replacement is done.
0x0024, 0x003C$<
  1. If namedCaptures isundefined, the replacement text is the String"$<".
  2. Else,
    1. Assert:Type(namedCaptures) is Object.
    2. Scan until the next > U+003E (GREATER-THAN SIGN).
    3. If none is found, the replacement text is the String"$<".
    4. Else,
      1. Let groupName be the enclosedsubstring.
      2. Let capture be ? Get(namedCaptures, groupName).
      3. If capture isundefined, replace the text through > with the empty String.
      4. Otherwise, replace the text through > with ? ToString(capture).
0x0024$ in any context that does not match any of the above.$

22.1.3.18 String.prototype.replaceAll ( searchValue, replaceValue )

When the replaceAll method is called with arguments searchValue and replaceValue, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If searchValue is neitherundefinednornull, then
    1. Let isRegExp be ? IsRegExp(searchValue).
    2. If isRegExp istrue, then
      1. Let flags be ? Get(searchValue,"flags").
      2. Perform ? RequireObjectCoercible(flags).
      3. If ? ToString(flags) does not contain"g", throw aTypeErrorexception.
    3. Let replacer be ? GetMethod(searchValue,@@replace).
    4. If replacer is notundefined, then
      1. Return ? Call(replacer, searchValue, « O, replaceValue »).
  3. Let string be ? ToString(O).
  4. Let searchString be ? ToString(searchValue).
  5. Let functionalReplace beIsCallable(replaceValue).
  6. If functionalReplace isfalse, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let searchLength be the length of searchString.
  8. Let advanceBy bemax(1, searchLength).
  9. Let matchPositions be a new emptyList.
  10. Let position be ! StringIndexOf(string, searchString, 0).
  11. Repeat, while position is not -1,
    1. Append position to the end of matchPositions.
    2. Set position to ! StringIndexOf(string, searchString, position + advanceBy).
  12. Let endOfLastMatch be 0.
  13. Let result be the empty String.
  14. For each element p of matchPositions, do
    1. Let preserved be thesubstringof string from endOfLastMatch to p.
    2. If functionalReplace istrue, then
      1. Let replacement be ? ToString(?Call(replaceValue,undefined, « searchString,𝔽(p), string »)).
    3. Else,
      1. Assert:Type(replaceValue) is String.
      2. Let captures be a new emptyList.
      3. Let replacement be ! GetSubstitution(searchString, string, p, captures,undefined, replaceValue).
    4. Set result to thestring-concatenationof result, preserved, and replacement.
    5. Set endOfLastMatch to p + searchLength.
  15. If endOfLastMatch < the length of string, then
    1. Set result to thestring-concatenationof result and thesubstringof string from endOfLastMatch.
  16. Return result.

22.1.3.19 String.prototype.search ( regexp )

When the search method is called with argument regexp, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If regexp is neitherundefinednornull, then
    1. Let searcher be ? GetMethod(regexp,@@search).
    2. If searcher is notundefined, then
      1. Return ? Call(searcher, regexp, « O »).
  3. Let string be ? ToString(O).
  4. Let rx be ? RegExpCreate(regexp,undefined).
  5. Return ? Invoke(rx,@@search, « string »).
Note

The search function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.20 String.prototype.slice ( start, end )

The slice method takes two arguments, start and end, and returns asubstringof the result of converting this object to a String, starting from index start and running to, but not including, index end (or through the end of the String if end isundefined). If start is negative, it is treated assourceLength + startwhere sourceLength is the length of the String. If end is negative, it is treated assourceLength + endwhere sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let len be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If intStart is -∞, let from be 0.
  6. Else if intStart < 0, let from bemax(len + intStart, 0).
  7. Else, let from bemin(intStart, len).
  8. If end isundefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
  9. If intEnd is -∞, let to be 0.
  10. Else if intEnd < 0, let to bemax(len + intEnd, 0).
  11. Else, let to bemin(intEnd, len).
  12. If fromto, return the empty String.
  13. Return thesubstringof S from from to to.
Note

The slice function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

22.1.3.21 String.prototype.split ( separator, limit )

Returns an Array object into which substrings of the result of converting this object to a String have been stored. The substrings are determined by searching from left to right for occurrences of separator; these occurrences are not part of any String in the returned array, but serve to divide up the String value. The value of separator may be a String of any length or it may be an object, such as a RegExp, that has a@@splitmethod.

When the split method is called, the following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. If separator is neitherundefinednornull, then
    1. Let splitter be ? GetMethod(separator,@@split).
    2. If splitter is notundefined, then
      1. Return ? Call(splitter, separator, « O, limit »).
  3. Let S be ? ToString(O).
  4. Let A be ! ArrayCreate(0).
  5. Let lengthA be 0.
  6. If limit isundefined, let lim be 232 - 1; else let lim be(?ToUint32(limit)).
  7. Let R be ? ToString(separator).
  8. If lim = 0, return A.
  9. If separator isundefined, then
    1. Perform ! CreateDataPropertyOrThrow(A,"0", S).
    2. Return A.
  10. Let s be the length of S.
  11. If s = 0, then
    1. If R is not the empty String, then
      1. Perform ! CreateDataPropertyOrThrow(A,"0", S).
    2. Return A.
  12. Let p be 0.
  13. Let q be p.
  14. Repeat, while qs,
    1. Let e beSplitMatch(S, q, R).
    2. If e isnot-matched, set q to q + 1.
    3. Else,
      1. Assert: e is a non-negativeintegers.
      2. If e = p, set q to q + 1.
      3. Else,
        1. Let T be thesubstringof S from p to q.
        2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
        3. Set lengthA to lengthA + 1.
        4. If lengthA = lim, return A.
        5. Set p to e.
        6. Set q to p.
  15. Let T be thesubstringof S from p to s.
  16. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
  17. Return A.
Note 1

The value of separator may be an empty String. In this case, separator does not match the emptysubstringat the beginning or end of the input String, nor does it match the emptysubstringat the end of the previous separator match. If separator is the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and eachsubstringcontains one code unit.

If thethisvalue is (or converts to) the empty String, the result depends on whether separator can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If separator isundefined, then the result array contains just one String, which is thethisvalue (converted to a String). If limit is notundefined, then the output array is truncated so that it contains no more than limit elements.

Note 2

The split function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.21.1 SplitMatch ( S, q, R )

The abstract operation SplitMatch takes arguments S (a String), q (a non-negativeinteger), and R (a String). It returns eithernot-matchedor the end index of a match. It performs the following steps when called:

  1. Let r be the number of code units in R.
  2. Let s be the number of code units in S.
  3. If q + r > s, returnnot-matched.
  4. If there exists anintegeri between 0 (inclusive) and r (exclusive) such that the code unit at index q + i within S is different from the code unit at index i within R, returnnot-matched.
  5. Return q + r.

22.1.3.22 String.prototype.startsWith ( searchString [ , position ] )

The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let isRegExp be ? IsRegExp(searchString).
  4. If isRegExp istrue, throw aTypeErrorexception.
  5. Let searchStr be ? ToString(searchString).
  6. Let len be the length of S.
  7. If position isundefined, let pos be 0; else let pos be ? ToIntegerOrInfinity(position).
  8. Let start be the result ofclampingpos between 0 and len.
  9. Let searchLength be the length of searchStr.
  10. If searchLength = 0, returntrue.
  11. Let end be start + searchLength.
  12. If end > len, returnfalse.
  13. Let substring be thesubstringof S from start to end.
  14. Return ! SameValueNonNumeric(substring, searchStr).
Note 1

This method returnstrueif the sequence of code units of searchString converted to a String is the same as the corresponding code units of this object (converted to a String) starting at index position. Otherwise returnsfalse.

Note 2

Throwing an exception if the first argument is a RegExp is specified in order to allow future editions to define extensions that allow such argument values.

Note 3

The startsWith function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.23 String.prototype.substring ( start, end )

The substring method takes two arguments, start and end, and returns asubstringof the result of converting this object to a String, starting from index start and running to, but not including, index end of the String (or through the end of the String if end isundefined). The result is a String value, not a String object.

If either argument isNaNor negative, it is replaced with zero; if either argument is larger than the length of the String, it is replaced with the length of the String.

If start is larger than end, they are swapped.

The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let len be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If end isundefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
  6. Let finalStart be the result ofclampingintStart between 0 and len.
  7. Let finalEnd be the result ofclampingintEnd between 0 and len.
  8. Let from bemin(finalStart, finalEnd).
  9. Let to bemax(finalStart, finalEnd).
  10. Return thesubstringof S from from to to.
Note

The substring function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.24 String.prototype.toLocaleLowerCase ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleLowerCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleLowerCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

This function works exactly the same as toLowerCase except that its result is intended to yield the correct result for thehost environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

Note

The toLocaleLowerCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.25 String.prototype.toLocaleUpperCase ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the toLocaleUpperCase method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleUpperCase method is used.

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

This function works exactly the same as toUpperCase except that its result is intended to yield the correct result for thehost environment's current locale, rather than a locale-independent result. There will only be a difference in the few cases (such as Turkish) where the rules for that language conflict with the regular Unicode case mappings.

The meaning of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

Note

The toLocaleUpperCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.26 String.prototype.toLowerCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4. The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let sText be ! StringToCodePoints(S).
  4. Let lowerText be the result of toLowercase(sText), according to the Unicode Default Case Conversion algorithm.
  5. Let L be ! CodePointsToString(lowerText).
  6. Return L.

The result must be derived according to the locale-insensitive case mappings in the Unicode Character Database (this explicitly includes not only the UnicodeData.txt file, but also all locale-insensitive mappings in the SpecialCasings.txt file that accompanies it).

Note 1

The case mapping of some code points may produce multiple code points. In this case the result String may not be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarily equal to s.toLowerCase().

Note 2

The toLowerCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.27 String.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. Return ? thisStringValue(thisvalue).
Note

For a String object, the toString method happens to return the same thing as the valueOf method.

22.1.3.28 String.prototype.toUpperCase ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

This function behaves in exactly the same way as String.prototype.toLowerCase, except that the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.

Note

The toUpperCase function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.29 String.prototype.trim ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

The following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? TrimString(S,start+end).
Note

The trim function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.29.1 TrimString ( string, where )

The abstract operation TrimString takes arguments string and where. It interprets string as a sequence of UTF-16 encoded code points, as described in6.1.4. It performs the following steps when called:

  1. Let str be ? RequireObjectCoercible(string).
  2. Let S be ? ToString(str).
  3. If where isstart, let T be the String value that is a copy of S with leading white space removed.
  4. Else if where isend, let T be the String value that is a copy of S with trailing white space removed.
  5. Else,
    1. Assert: where isstart+end.
    2. Let T be the String value that is a copy of S with both leading and trailing white space removed.
  6. Return T.

The definition of white space is the union ofWhiteSpaceandLineTerminator. When determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code unit sequences are interpreted as UTF-16 encoded code point sequences as specified in6.1.4.

22.1.3.30 String.prototype.trimEnd ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

The following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? TrimString(S,end).
Note

The trimEnd function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.31 String.prototype.trimStart ( )

This function interprets a String value as a sequence of UTF-16 encoded code points, as described in6.1.4.

The following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? TrimString(S,start).
Note

The trimStart function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore, it can be transferred to other kinds of objects for use as a method.

22.1.3.32 String.prototype.valueOf ( )

When the valueOf method is called, the following steps are taken:

  1. Return ? thisStringValue(thisvalue).

22.1.3.33 String.prototype [ @@iterator ] ( )

When the @@iterator method is called it returns an Iterator object (27.1.1.2) that iterates over the code points of a String value, returning each code point as a String value. The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let s be ? ToString(O).
  3. Let closure be a newAbstract Closurewith no parameters that captures s and performs the following steps when called:
    1. Let position be 0.
    2. Let len be the length of s.
    3. Repeat, while position < len,
      1. Let cp be ! CodePointAt(s, position).
      2. Let nextIndex be position + cp.[[CodeUnitCount]].
      3. Let resultString be thesubstringof s from position to nextIndex.
      4. Set position to nextIndex.
      5. Perform ? Yield(resultString).
    4. Returnundefined.
  4. Return ! CreateIteratorFromClosure(closure,"%StringIteratorPrototype%",%StringIteratorPrototype%).

The value of the"name"property of this function is"[Symbol.iterator]".

22.1.4 Properties of String Instances

String instances are String exotic objects and have the internal methods specified for such objects. String instances inherit properties from theString prototype object. String instances also have a [[StringData]] internal slot.

String instances have a"length"property, and a set of enumerable properties withinteger-indexed names.

22.1.4.1 length

The number of elements in the String value represented by this String object.

Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

22.1.5 String Iterator Objects

A String Iterator is an object, that represents a specific iteration over some specific String instance object. There is not a namedconstructorfor String Iterator objects. Instead, String iterator objects are created by calling certain methods of String instance objects.

22.1.5.1 The %StringIteratorPrototype% Object

The %StringIteratorPrototype% object:

  • has properties that are inherited by all String Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has the following properties:

22.1.5.1.1 %StringIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(thisvalue,empty,"%StringIteratorPrototype%").

22.1.5.1.2 %StringIteratorPrototype% [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"String Iterator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

22.2 RegExp (Regular Expression) Objects

A RegExp object contains a regular expression and the associated flags.

Note

The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5 programming language.

22.2.1 Patterns

The RegExpconstructorapplies the following grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an expansion ofPattern.

Syntax

Pattern[U, N]::Disjunction[?U, ?N]Disjunction[U, N]::Alternative[?U, ?N]Alternative[?U, ?N]|Disjunction[?U, ?N]Alternative[U, N]::[empty]Alternative[?U, ?N]Term[?U, ?N]Term[U, N]::Assertion[?U, ?N]Atom[?U, ?N]Atom[?U, ?N]QuantifierAssertion[U, N]::^$\b\B(?=Disjunction[?U, ?N])(?!Disjunction[?U, ?N])(?<=Disjunction[?U, ?N])(?<!Disjunction[?U, ?N])Quantifier::QuantifierPrefixQuantifierPrefix?QuantifierPrefix::*+?{DecimalDigits[~Sep]}{DecimalDigits[~Sep],}{DecimalDigits[~Sep],DecimalDigits[~Sep]}Atom[U, N]::PatternCharacter.\AtomEscape[?U, ?N]CharacterClass[?U](GroupSpecifier[?U]Disjunction[?U, ?N])(?:Disjunction[?U, ?N])SyntaxCharacter::one of^$\.*+?()[]{}|PatternCharacter::SourceCharacterbut notSyntaxCharacterAtomEscape[U, N]::DecimalEscapeCharacterClassEscape[?U]CharacterEscape[?U][+N]kGroupName[?U]CharacterEscape[U]::ControlEscapecControlLetter0[lookahead ∉DecimalDigit]HexEscapeSequenceRegExpUnicodeEscapeSequence[?U]IdentityEscape[?U]ControlEscape::one offnrtvControlLetter::one ofabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZGroupSpecifier[U]::[empty]?GroupName[?U]GroupName[U]::<RegExpIdentifierName[?U]>RegExpIdentifierName[U]::RegExpIdentifierStart[?U]RegExpIdentifierName[?U]RegExpIdentifierPart[?U]RegExpIdentifierStart[U]::UnicodeIDStart$_\RegExpUnicodeEscapeSequence[+U][~U]UnicodeLeadSurrogateUnicodeTrailSurrogateRegExpIdentifierPart[U]::UnicodeIDContinue$\RegExpUnicodeEscapeSequence[+U][~U]UnicodeLeadSurrogateUnicodeTrailSurrogate<ZWNJ><ZWJ>RegExpUnicodeEscapeSequence[U]::[+U]uHexLeadSurrogate\uHexTrailSurrogate[+U]uHexLeadSurrogate[+U]uHexTrailSurrogate[+U]uHexNonSurrogate[~U]uHex4Digits[+U]u{CodePoint}UnicodeLeadSurrogate::any Unicode code point in the inclusive range 0xD800 to 0xDBFFUnicodeTrailSurrogate::any Unicode code point in the inclusive range 0xDC00 to 0xDFFF

Each \uHexTrailSurrogatefor which the choice of associated uHexLeadSurrogateis ambiguous shall be associated with the nearest possible uHexLeadSurrogatethat would otherwise have no corresponding \uHexTrailSurrogate.

HexLeadSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis in the inclusive range 0xD800 to 0xDBFFHexTrailSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis in the inclusive range 0xDC00 to 0xDFFFHexNonSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis not in the inclusive range 0xD800 to 0xDFFFIdentityEscape[U]::[+U]SyntaxCharacter[+U]/[~U]SourceCharacterbut notUnicodeIDContinueDecimalEscape::NonZeroDigitDecimalDigits[~Sep]opt[lookahead ∉DecimalDigit]CharacterClassEscape[U]::dDsSwW[+U]p{UnicodePropertyValueExpression}[+U]P{UnicodePropertyValueExpression}UnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValueLoneUnicodePropertyNameOrValueUnicodePropertyName::UnicodePropertyNameCharactersUnicodePropertyNameCharacters::UnicodePropertyNameCharacterUnicodePropertyNameCharactersoptUnicodePropertyValue::UnicodePropertyValueCharactersLoneUnicodePropertyNameOrValue::UnicodePropertyValueCharactersUnicodePropertyValueCharacters::UnicodePropertyValueCharacterUnicodePropertyValueCharactersoptUnicodePropertyValueCharacter::UnicodePropertyNameCharacterDecimalDigitUnicodePropertyNameCharacter::ControlLetter_CharacterClass[U]::[[lookahead ≠^]ClassRanges[?U]][^ClassRanges[?U]]ClassRanges[U]::[empty]NonemptyClassRanges[?U]NonemptyClassRanges[U]::ClassAtom[?U]ClassAtom[?U]NonemptyClassRangesNoDash[?U]ClassAtom[?U]-ClassAtom[?U]ClassRanges[?U]NonemptyClassRangesNoDash[U]::ClassAtom[?U]ClassAtomNoDash[?U]NonemptyClassRangesNoDash[?U]ClassAtomNoDash[?U]-ClassAtom[?U]ClassRanges[?U]ClassAtom[U]::-ClassAtomNoDash[?U]ClassAtomNoDash[U]::SourceCharacterbut not one of\or]or-\ClassEscape[?U]ClassEscape[U]::b[+U]-CharacterClassEscape[?U]CharacterEscape[?U]Note

A number of productions in this section are given alternative definitions in sectionB.1.4.

22.2.1.1 Static Semantics: Early Errors

Note

This section is amended inB.1.4.1.

Pattern::DisjunctionQuantifierPrefix::{DecimalDigits,DecimalDigits}AtomEscape::kGroupNameAtomEscape::DecimalEscapeNonemptyClassRanges::ClassAtom-ClassAtomClassRangesNonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRangesRegExpIdentifierStart::\RegExpUnicodeEscapeSequenceRegExpIdentifierStart::UnicodeLeadSurrogateUnicodeTrailSurrogateRegExpIdentifierPart::\RegExpUnicodeEscapeSequenceRegExpIdentifierPart::UnicodeLeadSurrogateUnicodeTrailSurrogateUnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValueUnicodePropertyValueExpression::LoneUnicodePropertyNameOrValue
  • It is a Syntax Error if theListof Unicode code points that isSourceTextofLoneUnicodePropertyNameOrValueis not identical to aListof Unicode code points that is a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 61, nor a binary property or binary property alias listed in the “Property nameand aliases” column ofTable 60.

22.2.1.2 Static Semantics: CapturingGroupNumber

Note

This section is amended inB.1.4.1.

DecimalEscape::NonZeroDigit
  1. Return the MV ofNonZeroDigit.
DecimalEscape::NonZeroDigitDecimalDigits
  1. Let n be the number of code points inDecimalDigits.
  2. Return (the MV ofNonZeroDigit× 10n plus the MV ofDecimalDigits).

The definitions of “the MV ofNonZeroDigit” and “the MV ofDecimalDigits” are in12.8.3.

22.2.1.3 Static Semantics: IsCharacterClass

Note

This section is amended inB.1.4.2.

ClassAtom::-ClassAtomNoDash::SourceCharacterbut not one of\or]or-ClassEscape::bClassEscape::-ClassEscape::CharacterEscape
  1. Returnfalse.
ClassEscape::CharacterClassEscape
  1. Returntrue.

22.2.1.4 Static Semantics: CharacterValue

Note 1

This section is amended inB.1.4.3.

ClassAtom::-
  1. Return the code point value of U+002D (HYPHEN-MINUS).
ClassAtomNoDash::SourceCharacterbut not one of\or]or-
  1. Let ch be the code point matched bySourceCharacter.
  2. Return the code point value of ch.
ClassEscape::b
  1. Return the code point value of U+0008 (BACKSPACE).
ClassEscape::-
  1. Return the code point value of U+002D (HYPHEN-MINUS).
CharacterEscape::ControlEscape
  1. Return the code point value according toTable 58.
Table 58: ControlEscape Code Point Values
ControlEscapeCode Point ValueCode PointUnicode NameSymbol
t9U+0009CHARACTER TABULATION<HT>
n10U+000ALINE FEED (LF)<LF>
v11U+000BLINE TABULATION<VT>
f12U+000CFORM FEED (FF)<FF>
r13U+000DCARRIAGE RETURN (CR)<CR>
CharacterEscape::cControlLetter
  1. Let ch be the code point matched byControlLetter.
  2. Let i be ch's code point value.
  3. Return the remainder of dividing i by 32.
CharacterEscape::0[lookahead ∉DecimalDigit]
  1. Return the code point value of U+0000 (NULL).
Note 2

\0 represents the <NUL> character and cannot be followed by a decimal digit.

CharacterEscape::HexEscapeSequence
  1. Return the MV ofHexEscapeSequence.
RegExpUnicodeEscapeSequence::uHexLeadSurrogate\uHexTrailSurrogate
  1. Let lead be theCharacterValueofHexLeadSurrogate.
  2. Let trail be theCharacterValueofHexTrailSurrogate.
  3. Let cp beUTF16SurrogatePairToCodePoint(lead, trail).
  4. Return the code point value of cp.
RegExpUnicodeEscapeSequence::uHex4Digits
  1. Return the MV ofHex4Digits.
RegExpUnicodeEscapeSequence::u{CodePoint}
  1. Return the MV ofCodePoint.
HexLeadSurrogate::Hex4DigitsHexTrailSurrogate::Hex4DigitsHexNonSurrogate::Hex4Digits
  1. Return the MV ofHexDigits.
CharacterEscape::IdentityEscape
  1. Let ch be the code point matched byIdentityEscape.
  2. Return the code point value of ch.

22.2.1.5 Static Semantics: SourceText

UnicodePropertyNameCharacters::UnicodePropertyNameCharacterUnicodePropertyNameCharactersoptUnicodePropertyValueCharacters::UnicodePropertyValueCharacterUnicodePropertyValueCharactersopt
  1. Return theList, in source text order, of Unicode code points in the source text matched by this production.

22.2.1.6 Static Semantics: CapturingGroupName

RegExpIdentifierName::RegExpIdentifierStartRegExpIdentifierNameRegExpIdentifierPart
  1. Let idText be the source text matched byRegExpIdentifierName.
  2. Let idTextUnescaped be the result of replacing any occurrences of \RegExpUnicodeEscapeSequencein idText with the code point represented by theRegExpUnicodeEscapeSequence.
  3. Return ! CodePointsToString(idTextUnescaped).

22.2.2 Pattern Semantics

Note 1

This section is amended inB.1.4.4.

A regular expression pattern is converted into anAbstract Closureusing the process described below. An implementation is encouraged to use more efficient algorithms than the ones listed below, as long as the results are the same. TheAbstract Closureis used as the value of a RegExp object's [[RegExpMatcher]] internal slot.

APatternis either a BMP pattern or a Unicode pattern depending upon whether or not its associated flags contain a u. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either context, “character value” means the numeric value of the corresponding non-encoded code point.

The syntax and semantics ofPatternis defined as if the source code for thePatternwas aListofSourceCharactervalues where eachSourceCharactercorresponds to a Unicode code point. If a BMP pattern contains a non-BMPSourceCharacterthe entire pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of theList.

Note 2

For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character)Listconsisting of the single code point 0x1D11E. However, interpreted as a BMP pattern, it is first UTF-16 encoded to produce a two elementListconsisting of the code units 0xD834 and 0xDD1E.

Patterns are passed to the RegExpconstructoras ECMAScript String values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL G CLEF pattern, expressed as a String value, is a String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation of the string would be necessary to process it as a BMP pattern consisting of two pattern characters. However, to process it as a Unicode patternUTF16SurrogatePairToCodePointmust be used in producing aListwhose sole element is a single pattern character, the code point U+1D11E.

An implementation may not actually perform such translations to or from UTF-16, but the semantics of this specification requires that the result of pattern matching be as if such translations were performed.

22.2.2.1 Notation

The descriptions below use the following aliases:

  • Input is aListwhose elements are the characters of the String being matched by the regular expression pattern. Each character is either a code unit or a code point, depending upon the kind of pattern involved. The notation Input[n] means the nth character of Input, where n can range between 0 (inclusive) and InputLength (exclusive).
  • InputLength is the number of characters in Input.
  • NcapturingParens is the total number of left-capturing parentheses (i.e. the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes) in the pattern. A left-capturing parenthesis is any ( pattern character that is matched by the ( terminal of theAtom::(GroupSpecifierDisjunction)production.
  • DotAll istrueif the RegExp object's [[OriginalFlags]] internal slot contains"s"and otherwise isfalse.
  • IgnoreCase istrueif the RegExp object's [[OriginalFlags]] internal slot contains"i"and otherwise isfalse.
  • Multiline istrueif the RegExp object's [[OriginalFlags]] internal slot contains"m"and otherwise isfalse.
  • Unicode istrueif the RegExp object's [[OriginalFlags]] internal slot contains"u"and otherwise isfalse.
  • WordCharacters is the mathematical set that is the union of all sixty-three characters in"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_"(letters, numbers, and U+005F (LOW LINE) in the Unicode Basic Latin block) and all characters c for which c is not in that set butCanonicalize(c) is. WordCharacters cannot contain more than sixty-three characters unless Unicode and IgnoreCase are bothtrue.

Furthermore, the descriptions below use the following internal data structures:

  • A CharSet is a mathematical set of characters. When the Unicode flag istrue, “all characters” means the CharSet containing all code point values; otherwise “all characters” means the CharSet containing all code unit values.
  • A State is an ordered pair (endIndex, captures) where endIndex is anintegerand captures is aListof NcapturingParens values. States are used to represent partial match states in the regular expression matching algorithms. The endIndex is one plus the index of the last input character matched so far by the pattern, while captures holds the results of capturing parentheses. The nth element of captures is either aListof characters that represents the value obtained by the nth set of capturing parentheses orundefinedif the nth set of capturing parentheses hasn't been reached yet. Due to backtracking, many States may be in use at any time during the matching process.
  • A MatchResult is either a State or the special tokenfailurethat indicates that the match failed.
  • A Continuation is anAbstract Closurethat takes one State argument and returns a MatchResult result. The Continuation attempts to match the remaining portion (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. If the match succeeds, the Continuation returns the final State that it reached; if the match fails, the Continuation returnsfailure.
  • A Matcher is anAbstract Closurethat takes two arguments—a State and a Continuation—and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by the closure's captured values) of the pattern against Input, starting at the intermediate state given by its State argument. The Continuation argument should be a closure that matches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State, the Matcher then calls Continuation on that new State to test if the rest of the pattern can match as well. If it can, the Matcher returns the State returned by Continuation; if not, the Matcher may try different choices at its choice points, repeatedly calling Continuation until it either succeeds or all possibilities have been exhausted.

22.2.2.2 Pattern

The productionPattern::Disjunctionevaluates as follows:

  1. EvaluateDisjunctionwith 1 as its direction argument to obtain a Matcher m.
  2. Return a newAbstract Closurewith parameters (str, index) that captures m and performs the following steps when called:
    1. Assert:Type(str) is String.
    2. Assert: index is a non-negativeintegerwhich is ≤ the length of str.
    3. If Unicode istrue, let Input be ! StringToCodePoints(str). Otherwise, let Input be aListwhose elements are the code units that are the elements of str. Input will be used throughout the algorithms in22.2.2. Each element of Input is considered to be a character.
    4. Let InputLength be the number of characters contained in Input. This alias will be used throughout the algorithms in22.2.2.
    5. Let listIndex be the index into Input of the character that was obtained from element index of str.
    6. Let c be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a State.
      2. Return y.
    7. Let cap be aListof NcapturingParensundefinedvalues, indexed 1 through NcapturingParens.
    8. Let x be the State (listIndex, cap).
    9. Return m(x, c).
Note

A Pattern evaluates (“compiles”) to anAbstract Closurevalue.RegExpBuiltinExeccan then apply this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactly that offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms in22.2.2are designed so that compiling a pattern may throw aSyntaxErrorexception; on the other hand, once the pattern is successfully compiled, applying the resultingAbstract Closureto find a match in a String cannot throw an exception (except for anyimplementation-definedexceptions that can occur anywhere such as out-of-memory).

22.2.2.3 Disjunction

With parameter direction.

The productionDisjunction::Alternativeevaluates as follows:

  1. EvaluateAlternativewith argument direction to obtain a Matcher m.
  2. Return m.

The productionDisjunction::Alternative|Disjunctionevaluates as follows:

  1. EvaluateAlternativewith argument direction to obtain a Matcher m1.
  2. EvaluateDisjunctionwith argument direction to obtain a Matcher m2.
  3. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let r be m1(x, c).
    4. If r is notfailure, return r.
    5. Return m2(x, c).
Note

The | regular expression operator separates two alternatives. The pattern first tries to match the leftAlternative(followed by the sequel of the regular expression); if it fails, it tries to match the rightDisjunction(followed by the sequel of the regular expression). If the leftAlternative, the rightDisjunction, and the sequel all have choice points, all choices in the sequel are tried before moving on to the next choice in the leftAlternative. If choices in the leftAlternativeare exhausted, the rightDisjunctionis tried instead of the leftAlternative. Any capturing parentheses inside a portion of the pattern skipped by | produceundefinedvalues instead of Strings. Thus, for example,

/a|ab/.exec("abc")

returns the result"a"and not"ab". Moreover,

/((a)|(ab))((c)|(bc))/.exec("abc")

returns the array

["abc", "a", "a", undefined, "bc", undefined, "bc"]

and not

["abc", "ab", undefined, "ab", "c", "c", undefined]

The order in which the two alternatives are tried is independent of the value of direction.

22.2.2.4 Alternative

With parameter direction.

The productionAlternative::[empty]evaluates as follows:

  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Return c(x).

The productionAlternative::AlternativeTermevaluates as follows:

  1. EvaluateAlternativewith argument direction to obtain a Matcher m1.
  2. EvaluateTermwith argument direction to obtain a Matcher m2.
  3. If direction = 1, then
    1. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
      1. Assert: x is a State.
      2. Assert: c is a Continuation.
      3. Let d be a new Continuation with parameters (y) that captures c and m2 and performs the following steps when called:
        1. Assert: y is a State.
        2. Return m2(y, c).
      4. Return m1(x, d).
  4. Else,
    1. Assert: direction is -1.
    2. Return a new Matcher with parameters (x, c) that captures m1 and m2 and performs the following steps when called:
      1. Assert: x is a State.
      2. Assert: c is a Continuation.
      3. Let d be a new Continuation with parameters (y) that captures c and m1 and performs the following steps when called:
        1. Assert: y is a State.
        2. Return m1(y, c).
      4. Return m2(x, d).
Note

ConsecutiveTerms try to simultaneously match consecutive portions of Input. When direction = 1, if the leftAlternative, the rightTerm, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before moving on to the next choice in the rightTerm, and all choices in the rightTermare tried before moving on to the next choice in the leftAlternative. When direction = -1, the evaluation order ofAlternativeandTermare reversed.

22.2.2.5 Term

With parameter direction.

The productionTerm::Assertionevaluates as follows:

  1. Return the Matcher that is the result of evaluatingAssertion.
Note

The resulting Matcher is independent of direction.

The productionTerm::Atomevaluates as follows:

  1. Return the Matcher that is the result of evaluatingAtomwith argument direction.

The productionTerm::AtomQuantifierevaluates as follows:

  1. EvaluateAtomwith argument direction to obtain a Matcher m.
  2. EvaluateQuantifierto obtain the three results: a non-negativeintegermin, a non-negativeinteger(or +∞) max, and Boolean greedy.
  3. Assert: minmax.
  4. Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of thisTerm. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing thisTerm.
  5. Let parenCount be the number of left-capturing parentheses inAtom. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes enclosed byAtom.
  6. Return a new Matcher with parameters (x, c) that captures m, min, max, greedy, parenIndex, and parenCount and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Return ! RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount).

22.2.2.5.1 RepeatMatcher ( m, min, max, greedy, x, c, parenIndex, parenCount )

The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negativeinteger), max (a non-negativeintegeror +∞), greedy (a Boolean), x (a State), c (a Continuation), parenIndex (a non-negativeinteger), and parenCount (a non-negativeinteger). It performs the following steps when called:

  1. If max = 0, return c(x).
  2. Let d be a new Continuation with parameters (y) that captures m, min, max, greedy, x, c, parenIndex, and parenCount and performs the following steps when called:
    1. Assert: y is a State.
    2. If min = 0 and y's endIndex = x's endIndex, returnfailure.
    3. If min = 0, let min2 be 0; otherwise let min2 be min - 1.
    4. If max is +∞, let max2 be +∞; otherwise let max2 be max - 1.
    5. Return ! RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount).
  3. Let cap be a copy of x's capturesList.
  4. For eachintegerk such that parenIndex < k and kparenIndex + parenCount, set cap[k] toundefined.
  5. Let e be x's endIndex.
  6. Let xr be the State (e, cap).
  7. If min ≠ 0, return m(xr, d).
  8. If greedy isfalse, then
    1. Let z be c(x).
    2. If z is notfailure, return z.
    3. Return m(xr, d).
  9. Let z be m(xr, d).
  10. If z is notfailure, return z.
  11. Return c(x).
Note 1

AnAtomfollowed by aQuantifieris repeated the number of times specified by theQuantifier. AQuantifiercan be non-greedy, in which case theAtompattern is repeated as few times as possible while still matching the sequel, or it can be greedy, in which case theAtompattern is repeated as many times as possible while still matching the sequel. TheAtompattern is repeated rather than the input character sequence that it matches, so different repetitions of theAtomcan match different input substrings.

Note 2

If theAtomand the sequel of the regular expression all have choice points, theAtomis first matched as many (or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in the last repetition ofAtom. All choices in the last (nth) repetition ofAtomare tried before moving on to the next choice in the next-to-last (n - 1)st repetition ofAtom; at which point it may turn out that more or fewer repetitions ofAtomare now possible; these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in the (n - 1)st repetition ofAtomand so on.

Compare

/a[a-z]{2,4}/.exec("abcdefghi")

which returns"abcde"with

/a[a-z]{2,4}?/.exec("abcdefghi")

which returns"abc".

Consider also

/(aa|aabaac|ba|b|c)*/.exec("aabaac")

which, by the choice point ordering above, returns the array

["aaba", "ba"]

and not any of:

["aabaac", "aabaac"]
["aabaac", "c"]

The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisor of two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:

"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/, "$1")

which returns the gcd in unary notation"aaaaa".

Note 3

Step4of the RepeatMatcher clearsAtom's captures each timeAtomis repeated. We can see its behaviour in the regular expression

/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")

which returns the array

["zaacbbbcac", "z", "ac", "a", undefined, "c"]

and not

["zaacbbbcac", "z", "ac", "a", "bbb", "c"]

because each iteration of the outermost * clears all captured Strings contained in the quantifiedAtom, which in this case includes capture Strings numbered 2, 3, 4, and 5.

Note 4

Step2.bof the RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more expansions ofAtomthat match the empty character sequence are not considered for further repetitions. This prevents the regular expression engine from falling into an infinite loop on patterns such as:

/(a*)*/.exec("b")

or the slightly more complicated:

/(a*)b\1+/.exec("baaaac")

which returns the array

["b", ""]

22.2.2.6 Assertion

The productionAssertion::^evaluates as follows:

  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let e be x's endIndex.
    4. If e = 0, or if Multiline istrueand the character Input[e - 1] is one ofLineTerminator, then
      1. Return c(x).
    5. Returnfailure.
Note

Even when the y flag is used with a pattern, ^ always matches only at the beginning of Input, or (if Multiline istrue) at the beginning of a line.

The productionAssertion::$evaluates as follows:

  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let e be x's endIndex.
    4. If e = InputLength, or if Multiline istrueand the character Input[e] is one ofLineTerminator, then
      1. Return c(x).
    5. Returnfailure.

The productionAssertion::\bevaluates as follows:

  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let e be x's endIndex.
    4. Let a be ! IsWordChar(e - 1).
    5. Let b be ! IsWordChar(e).
    6. If a istrueand b isfalse, or if a isfalseand b istrue, return c(x).
    7. Returnfailure.

The productionAssertion::\Bevaluates as follows:

  1. Return a new Matcher with parameters (x, c) that captures nothing and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let e be x's endIndex.
    4. Let a be ! IsWordChar(e - 1).
    5. Let b be ! IsWordChar(e).
    6. If a istrueand b istrue, or if a isfalseand b isfalse, return c(x).
    7. Returnfailure.

The productionAssertion::(?=Disjunction)evaluates as follows:

  1. EvaluateDisjunctionwith 1 as its direction argument to obtain a Matcher m.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a State.
      2. Return y.
    4. Let r be m(x, d).
    5. If r isfailure, returnfailure.
    6. Let y be r's State.
    7. Let cap be y's capturesList.
    8. Let xe be x's endIndex.
    9. Let z be the State (xe, cap).
    10. Return c(z).

The productionAssertion::(?!Disjunction)evaluates as follows:

  1. EvaluateDisjunctionwith 1 as its direction argument to obtain a Matcher m.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a State.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is notfailure, returnfailure.
    6. Return c(x).

The productionAssertion::(?<=Disjunction)evaluates as follows:

  1. EvaluateDisjunctionwith -1 as its direction argument to obtain a Matcher m.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a State.
      2. Return y.
    4. Let r be m(x, d).
    5. If r isfailure, returnfailure.
    6. Let y be r's State.
    7. Let cap be y's capturesList.
    8. Let xe be x's endIndex.
    9. Let z be the State (xe, cap).
    10. Return c(z).

The productionAssertion::(?<!Disjunction)evaluates as follows:

  1. EvaluateDisjunctionwith -1 as its direction argument to obtain a Matcher m.
  2. Return a new Matcher with parameters (x, c) that captures m and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let d be a new Continuation with parameters (y) that captures nothing and performs the following steps when called:
      1. Assert: y is a State.
      2. Return y.
    4. Let r be m(x, d).
    5. If r is notfailure, returnfailure.
    6. Return c(x).

22.2.2.6.1 IsWordChar ( e )

The abstract operation IsWordChar takes argument e (aninteger). It performs the following steps when called:

  1. If e = -1 or e is InputLength, returnfalse.
  2. Let c be the character Input[e].
  3. If c is in WordCharacters, returntrue.
  4. Returnfalse.

22.2.2.7 Quantifier

The productionQuantifier::QuantifierPrefixevaluates as follows:

  1. EvaluateQuantifierPrefixto obtain the two results: anintegermin and aninteger(or +∞) max.
  2. Return the three results min, max, andtrue.

The productionQuantifier::QuantifierPrefix?evaluates as follows:

  1. EvaluateQuantifierPrefixto obtain the two results: anintegermin and aninteger(or +∞) max.
  2. Return the three results min, max, andfalse.

The productionQuantifierPrefix::*evaluates as follows:

  1. Return the two results 0 and +∞.

The productionQuantifierPrefix::+evaluates as follows:

  1. Return the two results 1 and +∞.

The productionQuantifierPrefix::?evaluates as follows:

  1. Return the two results 0 and 1.

The productionQuantifierPrefix::{DecimalDigits}evaluates as follows:

  1. Let i be the MV ofDecimalDigits(see12.8.3).
  2. Return the two results i and i.

The productionQuantifierPrefix::{DecimalDigits,}evaluates as follows:

  1. Let i be the MV ofDecimalDigits.
  2. Return the two results i and +∞.

The productionQuantifierPrefix::{DecimalDigits,DecimalDigits}evaluates as follows:

  1. Let i be the MV of the firstDecimalDigits.
  2. Let j be the MV of the secondDecimalDigits.
  3. Return the two results i and j.

22.2.2.8 Atom

With parameter direction.

The productionAtom::PatternCharacterevaluates as follows:

  1. Let ch be the character matched byPatternCharacter.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return ! CharacterSetMatcher(A,false, direction).

The productionAtom::.evaluates as follows:

  1. Let A be the CharSet of all characters.
  2. If DotAll is nottrue, then
    1. Remove from A all characters corresponding to a code point on the right-hand side of theLineTerminatorproduction.
  3. Return ! CharacterSetMatcher(A,false, direction).

The productionAtom::\AtomEscapeevaluates as follows:

  1. Return the Matcher that is the result of evaluatingAtomEscapewith argument direction.

The productionAtom::CharacterClassevaluates as follows:

  1. EvaluateCharacterClassto obtain a CharSet A and a Boolean invert.
  2. Return ! CharacterSetMatcher(A, invert, direction).

The productionAtom::(GroupSpecifierDisjunction)evaluates as follows:

  1. EvaluateDisjunctionwith argument direction to obtain a Matcher m.
  2. Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of thisAtom. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing thisAtom.
  3. Return a new Matcher with parameters (x, c) that captures direction, m, and parenIndex and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let d be a new Continuation with parameters (y) that captures x, c, direction, and parenIndex and performs the following steps when called:
      1. Assert: y is a State.
      2. Let cap be a copy of y's capturesList.
      3. Let xe be x's endIndex.
      4. Let ye be y's endIndex.
      5. If direction = 1, then
        1. Assert: xeye.
        2. Let s be aListwhose elements are the characters of Input at indices xe (inclusive) through ye (exclusive).
      6. Else,
        1. Assert: direction is -1.
        2. Assert: yexe.
        3. Let s be aListwhose elements are the characters of Input at indices ye (inclusive) through xe (exclusive).
      7. Set cap[parenIndex + 1] to s.
      8. Let z be the State (ye, cap).
      9. Return c(z).
    4. Return m(x, d).

The productionAtom::(?:Disjunction)evaluates as follows:

  1. Return the Matcher that is the result of evaluatingDisjunctionwith argument direction.

22.2.2.8.1 CharacterSetMatcher ( A, invert, direction )

The abstract operation CharacterSetMatcher takes arguments A (a CharSet), invert (a Boolean), and direction (1 or -1). It performs the following steps when called:

  1. Return a new Matcher with parameters (x, c) that captures A, invert, and direction and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let e be x's endIndex.
    4. Let f be e + direction.
    5. If f < 0 or f > InputLength, returnfailure.
    6. Let index bemin(e, f).
    7. Let ch be the character Input[index].
    8. Let cc beCanonicalize(ch).
    9. If there exists a member a of A such thatCanonicalize(a) is cc, let found betrue. Otherwise, let found befalse.
    10. If invert isfalseand found isfalse, returnfailure.
    11. If invert istrueand found istrue, returnfailure.
    12. Let cap be x's capturesList.
    13. Let y be the State (f, cap).
    14. Return c(y).

22.2.2.8.2 Canonicalize ( ch )

The abstract operation Canonicalize takes argument ch (a character). It performs the following steps when called:

  1. If Unicode istrueand IgnoreCase istrue, then
    1. If the file CaseFolding.txt of the Unicode Character Database provides a simple or common case folding mapping for ch, return the result of applying that mapping to ch.
    2. Return ch.
  2. If IgnoreCase isfalse, return ch.
  3. Assert: ch is a UTF-16 code unit.
  4. Let cp be the code point whose numeric value is that of ch.
  5. Let u be the result of toUppercase(« cp »), according to the Unicode Default Case Conversion algorithm.
  6. Let uStr be ! CodePointsToString(u).
  7. If uStr does not consist of a single code unit, return ch.
  8. Let cu be uStr's single code unit element.
  9. If the numeric value of ch ≥ 128 and the numeric value of cu < 128, return ch.
  10. Return cu.
Note 1

Parentheses of the form (Disjunction) serve both to group the components of theDisjunctionpattern together and to save the result of the match. The result can be used either in a backreference (\ followed by a non-zero decimal number), referenced in a replace String, or returned as part of an array from the regular expression matchingAbstract Closure. To inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.

Note 2

The form (?=Disjunction) specifies a zero-width positive lookahead. In order for it to succeed, the pattern insideDisjunctionmust match at the current position, but the current position is not advanced before matching the sequel. IfDisjunctioncan match at the current position in several ways, only the first one is tried. Unlike other regular expression operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when theDisjunctioncontains capturing parentheses and the sequel of the pattern contains backreferences to those captures.

For example,

/(?=(a+))/.exec("baaabac")

matches the empty String immediately after the first b and therefore returns the array:

["", "aaa"]

To illustrate the lack of backtracking into the lookahead, consider:

/(?=(a+))a*b\1/.exec("baaabac")

This expression returns

["aba", "a"]

and not:

["aaaba", "a"]
Note 3

The form (?!Disjunction) specifies a zero-width negative lookahead. In order for it to succeed, the pattern insideDisjunctionmust fail to match at the current position. The current position is not advanced before matching the sequel.Disjunctioncan contain capturing parentheses, but backreferences to them only make sense from withinDisjunctionitself. Backreferences to these capturing parentheses from elsewhere in the pattern always returnundefinedbecause the negative lookahead must fail for the pattern to succeed. For example,

/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")

looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) and a c. The second \2 is outside the negative lookahead, so it matches againstundefinedand therefore always succeeds. The whole expression returns the array:

["baaabaac", "ba", undefined, "abaac"]
Note 4

In case-insignificant matches when Unicode istrue, all characters are implicitly case-folded using the simple mapping provided by the Unicode standard immediately before they are compared. The simple mapping always maps to a single code point, so it does not map, for example, ß (U+00DF) to SS. It may however map a code point outside the Basic Latin range to a character within, for example, ſ (U+017F) to s. Such characters are not mapped if Unicode isfalse. This prevents Unicode code points such as U+017F and U+212A from matching regular expressions such as /[a-z]/i, but they will match /[a-z]/ui.

22.2.2.8.3 UnicodeMatchProperty ( p )

The abstract operation UnicodeMatchProperty takes argument p (aListof Unicode code points). It performs the following steps when called:

  1. Assert: p is aListof Unicode code points that is identical to aListof Unicode code points that is a Unicodeproperty nameor property alias listed in the “Property nameand aliases” column ofTable 59orTable 60.
  2. Let c be the canonicalproperty nameof p as given in the “Canonicalproperty name” column of the corresponding row.
  3. Return theListof Unicode code points of c.

Implementations must support the Unicode property names and aliases listed inTable 59andTable 60. To ensure interoperability, implementations must not support any other property names or aliases.

Note 1

For example, Script_Extensions (property name) and scx (property alias) are valid, but script_extensions or Scx aren't.

Note 2

The listed properties form a superset of what UTS18 RL1.2 requires.

Table 59: Non-binary Unicode property aliases and their canonical property names
Property nameand aliasesCanonicalproperty name
General_CategoryGeneral_Category
gc
ScriptScript
sc
Script_ExtensionsScript_Extensions
scx
Table 60: Binary Unicode property aliases and their canonical property names
Property nameand aliasesCanonicalproperty name
ASCIIASCII
ASCII_Hex_DigitASCII_Hex_Digit
AHex
AlphabeticAlphabetic
Alpha
AnyAny
AssignedAssigned
Bidi_ControlBidi_Control
Bidi_C
Bidi_MirroredBidi_Mirrored
Bidi_M
Case_IgnorableCase_Ignorable
CI
CasedCased
Changes_When_CasefoldedChanges_When_Casefolded
CWCF
Changes_When_CasemappedChanges_When_Casemapped
CWCM
Changes_When_LowercasedChanges_When_Lowercased
CWL
Changes_When_NFKC_CasefoldedChanges_When_NFKC_Casefolded
CWKCF
Changes_When_TitlecasedChanges_When_Titlecased
CWT
Changes_When_UppercasedChanges_When_Uppercased
CWU
DashDash
Default_Ignorable_Code_PointDefault_Ignorable_Code_Point
DI
DeprecatedDeprecated
Dep
DiacriticDiacritic
Dia
EmojiEmoji
Emoji_ComponentEmoji_Component
EComp
Emoji_ModifierEmoji_Modifier
EMod
Emoji_Modifier_BaseEmoji_Modifier_Base
EBase
Emoji_PresentationEmoji_Presentation
EPres
Extended_PictographicExtended_Pictographic
ExtPict
ExtenderExtender
Ext
Grapheme_BaseGrapheme_Base
Gr_Base
Grapheme_ExtendGrapheme_Extend
Gr_Ext
Hex_DigitHex_Digit
Hex
IDS_Binary_OperatorIDS_Binary_Operator
IDSB
IDS_Trinary_OperatorIDS_Trinary_Operator
IDST
ID_ContinueID_Continue
IDC
ID_StartID_Start
IDS
IdeographicIdeographic
Ideo
Join_ControlJoin_Control
Join_C
Logical_Order_ExceptionLogical_Order_Exception
LOE
LowercaseLowercase
Lower
MathMath
Noncharacter_Code_PointNoncharacter_Code_Point
NChar
Pattern_SyntaxPattern_Syntax
Pat_Syn
Pattern_White_SpacePattern_White_Space
Pat_WS
Quotation_MarkQuotation_Mark
QMark
RadicalRadical
Regional_IndicatorRegional_Indicator
RI
Sentence_TerminalSentence_Terminal
STerm
Soft_DottedSoft_Dotted
SD
Terminal_PunctuationTerminal_Punctuation
Term
Unified_IdeographUnified_Ideograph
UIdeo
UppercaseUppercase
Upper
Variation_SelectorVariation_Selector
VS
White_SpaceWhite_Space
space
XID_ContinueXID_Continue
XIDC
XID_StartXID_Start
XIDS

22.2.2.8.4 UnicodeMatchPropertyValue ( p, v )

The abstract operation UnicodeMatchPropertyValue takes arguments p (aListof Unicode code points) and v (aListof Unicode code points). It performs the following steps when called:

  1. Assert: p is aListof Unicode code points that is identical to aListof Unicode code points that is a canonical, unaliased Unicodeproperty namelisted in the “Canonicalproperty name” column ofTable 59.
  2. Assert: v is aListof Unicode code points that is identical to aListof Unicode code points that is a property value or property value alias for Unicode property p listed in the “Property value and aliases” column ofTable 61orTable 62.
  3. Let value be the canonical property value of v as given in the “Canonical property value” column of the corresponding row.
  4. Return theListof Unicode code points of value.

Implementations must support the Unicode property value names and aliases listed inTable 61andTable 62. To ensure interoperability, implementations must not support any other property value names or aliases.

Note 1

For example, Xpeo and Old_Persian are valid Script_Extensions values, but xpeo and Old Persian aren't.

Note 2

This algorithm differs from the matching rules for symbolic values listed in UAX44: case,white space, U+002D (HYPHEN-MINUS), and U+005F (LOW LINE) are not ignored, and the Is prefix is not supported.

Table 61: Value aliases and canonical values for the Unicode property General_Category
Property value and aliasesCanonical property value
Cased_LetterCased_Letter
LC
Close_PunctuationClose_Punctuation
Pe
Connector_PunctuationConnector_Punctuation
Pc
ControlControl
Cc
cntrl
Currency_SymbolCurrency_Symbol
Sc
Dash_PunctuationDash_Punctuation
Pd
Decimal_NumberDecimal_Number
Nd
digit
Enclosing_MarkEnclosing_Mark
Me
Final_PunctuationFinal_Punctuation
Pf
FormatFormat
Cf
Initial_PunctuationInitial_Punctuation
Pi
LetterLetter
L
Letter_NumberLetter_Number
Nl
Line_SeparatorLine_Separator
Zl
Lowercase_LetterLowercase_Letter
Ll
MarkMark
M
Combining_Mark
Math_SymbolMath_Symbol
Sm
Modifier_LetterModifier_Letter
Lm
Modifier_SymbolModifier_Symbol
Sk
Nonspacing_MarkNonspacing_Mark
Mn
NumberNumber
N
Open_PunctuationOpen_Punctuation
Ps
OtherOther
C
Other_LetterOther_Letter
Lo
Other_NumberOther_Number
No
Other_PunctuationOther_Punctuation
Po
Other_SymbolOther_Symbol
So
Paragraph_SeparatorParagraph_Separator
Zp
Private_UsePrivate_Use
Co
PunctuationPunctuation
P
punct
SeparatorSeparator
Z
Space_SeparatorSpace_Separator
Zs
Spacing_MarkSpacing_Mark
Mc
SurrogateSurrogate
Cs
SymbolSymbol
S
Titlecase_LetterTitlecase_Letter
Lt
UnassignedUnassigned
Cn
Uppercase_LetterUppercase_Letter
Lu
Table 62: Value aliases and canonical values for the Unicode properties Script and Script_Extensions
Property value and aliasesCanonical property value
AdlamAdlam
Adlm
AhomAhom
Anatolian_HieroglyphsAnatolian_Hieroglyphs
Hluw
ArabicArabic
Arab
ArmenianArmenian
Armn
AvestanAvestan
Avst
BalineseBalinese
Bali
BamumBamum
Bamu
Bassa_VahBassa_Vah
Bass
BatakBatak
Batk
BengaliBengali
Beng
BhaiksukiBhaiksuki
Bhks
BopomofoBopomofo
Bopo
BrahmiBrahmi
Brah
BrailleBraille
Brai
BugineseBuginese
Bugi
BuhidBuhid
Buhd
Canadian_AboriginalCanadian_Aboriginal
Cans
CarianCarian
Cari
Caucasian_AlbanianCaucasian_Albanian
Aghb
ChakmaChakma
Cakm
ChamCham
ChorasmianChorasmian
Chrs
CherokeeCherokee
Cher
CommonCommon
Zyyy
CopticCoptic
Copt
Qaac
CuneiformCuneiform
Xsux
CypriotCypriot
Cprt
CyrillicCyrillic
Cyrl
DeseretDeseret
Dsrt
DevanagariDevanagari
Deva
Dives_AkuruDives_Akuru
Diak
DograDogra
Dogr
DuployanDuployan
Dupl
Egyptian_HieroglyphsEgyptian_Hieroglyphs
Egyp
ElbasanElbasan
Elba
ElymaicElymaic
Elym
EthiopicEthiopic
Ethi
GeorgianGeorgian
Geor
GlagoliticGlagolitic
Glag
GothicGothic
Goth
GranthaGrantha
Gran
GreekGreek
Grek
GujaratiGujarati
Gujr
Gunjala_GondiGunjala_Gondi
Gong
GurmukhiGurmukhi
Guru
HanHan
Hani
HangulHangul
Hang
Hanifi_RohingyaHanifi_Rohingya
Rohg
HanunooHanunoo
Hano
HatranHatran
Hatr
HebrewHebrew
Hebr
HiraganaHiragana
Hira
Imperial_AramaicImperial_Aramaic
Armi
InheritedInherited
Zinh
Qaai
Inscriptional_PahlaviInscriptional_Pahlavi
Phli
Inscriptional_ParthianInscriptional_Parthian
Prti
JavaneseJavanese
Java
KaithiKaithi
Kthi
KannadaKannada
Knda
KatakanaKatakana
Kana
Kayah_LiKayah_Li
Kali
KharoshthiKharoshthi
Khar
Khitan_Small_ScriptKhitan_Small_Script
Kits
KhmerKhmer
Khmr
KhojkiKhojki
Khoj
KhudawadiKhudawadi
Sind
LaoLao
Laoo
LatinLatin
Latn
LepchaLepcha
Lepc
LimbuLimbu
Limb
Linear_ALinear_A
Lina
Linear_BLinear_B
Linb
LisuLisu
LycianLycian
Lyci
LydianLydian
Lydi
MahajaniMahajani
Mahj
MakasarMakasar
Maka
MalayalamMalayalam
Mlym
MandaicMandaic
Mand
ManichaeanManichaean
Mani
MarchenMarchen
Marc
MedefaidrinMedefaidrin
Medf
Masaram_GondiMasaram_Gondi
Gonm
Meetei_MayekMeetei_Mayek
Mtei
Mende_KikakuiMende_Kikakui
Mend
Meroitic_CursiveMeroitic_Cursive
Merc
Meroitic_HieroglyphsMeroitic_Hieroglyphs
Mero
MiaoMiao
Plrd
ModiModi
MongolianMongolian
Mong
MroMro
Mroo
MultaniMultani
Mult
MyanmarMyanmar
Mymr
NabataeanNabataean
Nbat
NandinagariNandinagari
Nand
New_Tai_LueNew_Tai_Lue
Talu
NewaNewa
NkoNko
Nkoo
NushuNushu
Nshu
Nyiakeng_Puachue_HmongNyiakeng_Puachue_Hmong
Hmnp
OghamOgham
Ogam
Ol_ChikiOl_Chiki
Olck
Old_HungarianOld_Hungarian
Hung
Old_ItalicOld_Italic
Ital
Old_North_ArabianOld_North_Arabian
Narb
Old_PermicOld_Permic
Perm
Old_PersianOld_Persian
Xpeo
Old_SogdianOld_Sogdian
Sogo
Old_South_ArabianOld_South_Arabian
Sarb
Old_TurkicOld_Turkic
Orkh
OriyaOriya
Orya
OsageOsage
Osge
OsmanyaOsmanya
Osma
Pahawh_HmongPahawh_Hmong
Hmng
PalmyrenePalmyrene
Palm
Pau_Cin_HauPau_Cin_Hau
Pauc
Phags_PaPhags_Pa
Phag
PhoenicianPhoenician
Phnx
Psalter_PahlaviPsalter_Pahlavi
Phlp
RejangRejang
Rjng
RunicRunic
Runr
SamaritanSamaritan
Samr
SaurashtraSaurashtra
Saur
SharadaSharada
Shrd
ShavianShavian
Shaw
SiddhamSiddham
Sidd
SignWritingSignWriting
Sgnw
SinhalaSinhala
Sinh
SogdianSogdian
Sogd
Sora_SompengSora_Sompeng
Sora
SoyomboSoyombo
Soyo
SundaneseSundanese
Sund
Syloti_NagriSyloti_Nagri
Sylo
SyriacSyriac
Syrc
TagalogTagalog
Tglg
TagbanwaTagbanwa
Tagb
Tai_LeTai_Le
Tale
Tai_ThamTai_Tham
Lana
Tai_VietTai_Viet
Tavt
TakriTakri
Takr
TamilTamil
Taml
TangutTangut
Tang
TeluguTelugu
Telu
ThaanaThaana
Thaa
ThaiThai
TibetanTibetan
Tibt
TifinaghTifinagh
Tfng
TirhutaTirhuta
Tirh
UgariticUgaritic
Ugar
VaiVai
Vaii
WanchoWancho
Wcho
Warang_CitiWarang_Citi
Wara
YezidiYezidi
Yezi
YiYi
Yiii
Zanabazar_SquareZanabazar_Square
Zanb

22.2.2.9 AtomEscape

With parameter direction.

The productionAtomEscape::DecimalEscapeevaluates as follows:

  1. EvaluateDecimalEscapeto obtain anintegern.
  2. Assert: nNcapturingParens.
  3. Return ! BackreferenceMatcher(n, direction).

The productionAtomEscape::CharacterEscapeevaluates as follows:

  1. EvaluateCharacterEscapeto obtain a character ch.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return ! CharacterSetMatcher(A,false, direction).

The productionAtomEscape::CharacterClassEscapeevaluates as follows:

  1. EvaluateCharacterClassEscapeto obtain a CharSet A.
  2. Return ! CharacterSetMatcher(A,false, direction).
Note

An escape sequence of the form \ followed by a non-zero decimal number n matches the result of the nth set of capturing parentheses (22.2.2.1). It is an error if the regular expression has fewer than n capturing parentheses. If the regular expression has n or more capturing parentheses but the nth one isundefinedbecause it has not captured anything, then the backreference always succeeds.

The productionAtomEscape::kGroupNameevaluates as follows:

  1. Search the enclosingPatternfor an instance of aGroupSpecifiercontaining aRegExpIdentifierNamewhich has aCapturingGroupNameequal to theCapturingGroupNameof theRegExpIdentifierNamecontained inGroupName.
  2. Assert: A unique suchGroupSpecifieris found.
  3. Let parenIndex be the number of left-capturing parentheses in the entire regular expression that occur to the left of the locatedGroupSpecifier. This is the total number ofAtom::(GroupSpecifierDisjunction)Parse Nodes prior to or enclosing the locatedGroupSpecifier, including its immediately enclosingAtom.
  4. Return ! BackreferenceMatcher(parenIndex, direction).

22.2.2.9.1 BackreferenceMatcher ( n, direction )

The abstract operation BackreferenceMatcher takes arguments n (a positiveinteger) and direction (1 or -1). It performs the following steps when called:

  1. Assert: n ≥ 1.
  2. Return a new Matcher with parameters (x, c) that captures n and direction and performs the following steps when called:
    1. Assert: x is a State.
    2. Assert: c is a Continuation.
    3. Let cap be x's capturesList.
    4. Let s be cap[n].
    5. If s isundefined, return c(x).
    6. Let e be x's endIndex.
    7. Let len be the number of elements in s.
    8. Let f be e + direction × len.
    9. If f < 0 or f > InputLength, returnfailure.
    10. Let g bemin(e, f).
    11. If there exists anintegeri between 0 (inclusive) and len (exclusive) such thatCanonicalize(s[i]) is not the same character value asCanonicalize(Input[g + i]), returnfailure.
    12. Let y be the State (f, cap).
    13. Return c(y).

22.2.2.10 CharacterEscape

TheCharacterEscapeproductions evaluate as follows:

CharacterEscape::ControlEscapecControlLetter0[lookahead ∉DecimalDigit]HexEscapeSequenceRegExpUnicodeEscapeSequenceIdentityEscape
  1. Let cv be theCharacterValueof thisCharacterEscape.
  2. Return the character whose character value is cv.

22.2.2.11 DecimalEscape

TheDecimalEscapeproductions evaluate as follows:

DecimalEscape::NonZeroDigitDecimalDigitsopt
  1. Return theCapturingGroupNumberof thisDecimalEscape.
Note

If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to be a backreference. It is an error if n is greater than the total number of left-capturing parentheses in the entire regular expression.

22.2.2.12 CharacterClassEscape

The productionCharacterClassEscape::devaluates as follows:

  1. Return the ten-element CharSet containing the characters 0 through 9 inclusive.

The productionCharacterClassEscape::Devaluates as follows:

  1. Return the CharSet containing all characters not in the CharSet returned byCharacterClassEscape::d.

The productionCharacterClassEscape::sevaluates as follows:

  1. Return the CharSet containing all characters corresponding to a code point on the right-hand side of theWhiteSpaceorLineTerminatorproductions.

The productionCharacterClassEscape::Sevaluates as follows:

  1. Return the CharSet containing all characters not in the CharSet returned byCharacterClassEscape::s.

The productionCharacterClassEscape::wevaluates as follows:

  1. Return WordCharacters.

The productionCharacterClassEscape::Wevaluates as follows:

  1. Return the CharSet containing all characters not in the CharSet returned byCharacterClassEscape::w.

The productionCharacterClassEscape::p{UnicodePropertyValueExpression}evaluates as follows:

  1. Return the CharSet containing all Unicode code points included in the CharSet returned byUnicodePropertyValueExpression.

The productionCharacterClassEscape::P{UnicodePropertyValueExpression}evaluates as follows:

  1. Return the CharSet containing all Unicode code points not included in the CharSet returned byUnicodePropertyValueExpression.

The productionUnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValueevaluates as follows:

  1. Let ps beSourceTextofUnicodePropertyName.
  2. Let p be ! UnicodeMatchProperty(ps).
  3. Assert: p is a Unicodeproperty nameor property alias listed in the “Property nameand aliases” column ofTable 59.
  4. Let vs beSourceTextofUnicodePropertyValue.
  5. Let v be ! UnicodeMatchPropertyValue(p, vs).
  6. Return the CharSet containing all Unicode code points whose character database definition includes the property p with value v.

The productionUnicodePropertyValueExpression::LoneUnicodePropertyNameOrValueevaluates as follows:

  1. Let s beSourceTextofLoneUnicodePropertyNameOrValue.
  2. If ! UnicodeMatchPropertyValue(General_Category, s) is identical to aListof Unicode code points that is the name of a Unicode general category or general category alias listed in the “Property value and aliases” column ofTable 61, then
    1. Return the CharSet containing all Unicode code points whose character database definition includes the property “General_Category” with value s.
  3. Let p be ! UnicodeMatchProperty(s).
  4. Assert: p is a binary Unicode property or binary property alias listed in the “Property nameand aliases” column ofTable 60.
  5. Return the CharSet containing all Unicode code points whose character database definition includes the property p with value “True”.

22.2.2.13 CharacterClass

The productionCharacterClass::[ClassRanges]evaluates as follows:

  1. EvaluateClassRangesto obtain a CharSet A.
  2. Return the two results A andfalse.

The productionCharacterClass::[^ClassRanges]evaluates as follows:

  1. EvaluateClassRangesto obtain a CharSet A.
  2. Return the two results A andtrue.

22.2.2.14 ClassRanges

The productionClassRanges::[empty]evaluates as follows:

  1. Return the empty CharSet.

The productionClassRanges::NonemptyClassRangesevaluates as follows:

  1. Return the CharSet that is the result of evaluatingNonemptyClassRanges.

22.2.2.15 NonemptyClassRanges

The productionNonemptyClassRanges::ClassAtomevaluates as follows:

  1. Return the CharSet that is the result of evaluatingClassAtom.

The productionNonemptyClassRanges::ClassAtomNonemptyClassRangesNoDashevaluates as follows:

  1. EvaluateClassAtomto obtain a CharSet A.
  2. EvaluateNonemptyClassRangesNoDashto obtain a CharSet B.
  3. Return the union of CharSets A and B.

The productionNonemptyClassRanges::ClassAtom-ClassAtomClassRangesevaluates as follows:

  1. Evaluate the firstClassAtomto obtain a CharSet A.
  2. Evaluate the secondClassAtomto obtain a CharSet B.
  3. EvaluateClassRangesto obtain a CharSet C.
  4. Let D be ! CharacterRange(A, B).
  5. Return the union of D and C.

22.2.2.15.1 CharacterRange ( A, B )

The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:

  1. Assert: A and B each contain exactly one character.
  2. Let a be the one character in CharSet A.
  3. Let b be the one character in CharSet B.
  4. Let i be the character value of character a.
  5. Let j be the character value of character b.
  6. Assert: ij.
  7. Return the CharSet containing all characters with a character value greater than or equal to i and less than or equal to j.

22.2.2.16 NonemptyClassRangesNoDash

The productionNonemptyClassRangesNoDash::ClassAtomevaluates as follows:

  1. Return the CharSet that is the result of evaluatingClassAtom.

The productionNonemptyClassRangesNoDash::ClassAtomNoDashNonemptyClassRangesNoDashevaluates as follows:

  1. EvaluateClassAtomNoDashto obtain a CharSet A.
  2. EvaluateNonemptyClassRangesNoDashto obtain a CharSet B.
  3. Return the union of CharSets A and B.

The productionNonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRangesevaluates as follows:

  1. EvaluateClassAtomNoDashto obtain a CharSet A.
  2. EvaluateClassAtomto obtain a CharSet B.
  3. EvaluateClassRangesto obtain a CharSet C.
  4. Let D be ! CharacterRange(A, B).
  5. Return the union of D and C.
Note 1

ClassRangescan expand into a singleClassAtomand/or ranges of twoClassAtomseparated by dashes. In the latter case theClassRangesincludes all characters between the firstClassAtomand the secondClassAtom, inclusive; an error occurs if eitherClassAtomdoes not represent a single character (for example, if one is \w) or if the firstClassAtom's character value is greater than the secondClassAtom's character value.

Note 2

Even if the pattern ignores case, the case of the two ends of a range is significant in determining which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while the pattern /[E-f]/i matches all upper and lower-case letters in the Unicode Basic Latin block as well as the symbols [, \, ], ^, _, and `.

Note 3

A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last character ofClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

22.2.2.17 ClassAtom

The productionClassAtom::-evaluates as follows:

  1. Return the CharSet containing the single character - U+002D (HYPHEN-MINUS).

The productionClassAtom::ClassAtomNoDashevaluates as follows:

  1. Return the CharSet that is the result of evaluatingClassAtomNoDash.

22.2.2.18 ClassAtomNoDash

The productionClassAtomNoDash::SourceCharacterbut not one of\or]or-evaluates as follows:

  1. Return the CharSet containing the character matched bySourceCharacter.

The productionClassAtomNoDash::\ClassEscapeevaluates as follows:

  1. Return the CharSet that is the result of evaluatingClassEscape.

22.2.2.19 ClassEscape

TheClassEscapeproductions evaluate as follows:

ClassEscape::bClassEscape::-ClassEscape::CharacterEscape
  1. Let cv be theCharacterValueof thisClassEscape.
  2. Let c be the character whose character value is cv.
  3. Return the CharSet containing the single character c.
ClassEscape::CharacterClassEscape
  1. Return the CharSet that is the result of evaluatingCharacterClassEscape.
Note

AClassAtomcan use any of the escape sequences that are allowed in the rest of the regular expression except for \b, \B, and backreferences. Inside aCharacterClass, \b means the backspace character, while \B and backreferences raise errors. Using a backreference inside aClassAtomcauses an error.

22.2.3 The RegExp Constructor

The RegExpconstructor:

  • is %RegExp%.
  • is the initial value of the"RegExp"property of theglobal object.
  • creates and initializes a new RegExp object when called as a function rather than as aconstructor. Thus the function call RegExp(…) is equivalent to the object creation expression new RegExp(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified RegExp behaviour must include a super call to the RegExpconstructorto create and initialize subclass instances with the necessary internal slots.

22.2.3.1 RegExp ( pattern, flags )

The following steps are taken:

  1. Let patternIsRegExp be ? IsRegExp(pattern).
  2. If NewTarget isundefined, then
    1. Let newTarget be theactive function object.
    2. If patternIsRegExp istrueand flags isundefined, then
      1. Let patternConstructor be ? Get(pattern,"constructor").
      2. IfSameValue(newTarget, patternConstructor) istrue, return pattern.
  3. Else, let newTarget be NewTarget.
  4. IfType(pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then
    1. Let P be pattern.[[OriginalSource]].
    2. If flags isundefined, let F be pattern.[[OriginalFlags]].
    3. Else, let F be flags.
  5. Else if patternIsRegExp istrue, then
    1. Let P be ? Get(pattern,"source").
    2. If flags isundefined, then
      1. Let F be ? Get(pattern,"flags").
    3. Else, let F be flags.
  6. Else,
    1. Let P be pattern.
    2. Let F be flags.
  7. Let O be ? RegExpAlloc(newTarget).
  8. Return ? RegExpInitialize(O, P, F).
Note

If pattern is supplied using aStringLiteral, the usual escape sequence substitutions are performed before the String is processed by RegExp. If pattern must contain an escape sequence to be recognized by RegExp, any U+005C (REVERSE SOLIDUS) code points must be escaped within theStringLiteralto prevent them being removed when the contents of theStringLiteralare formed.

22.2.3.2 Abstract Operations for the RegExp Constructor

22.2.3.2.1 RegExpAlloc ( newTarget )

The abstract operation RegExpAlloc takes argument newTarget. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(newTarget,"%RegExp.prototype%", « [[RegExpMatcher]], [[OriginalSource]], [[OriginalFlags]] »).
  2. Perform ! DefinePropertyOrThrow(obj,"lastIndex", PropertyDescriptor { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}).
  3. Return obj.

22.2.3.2.2 RegExpInitialize ( obj, pattern, flags )

The abstract operation RegExpInitialize takes arguments obj, pattern, and flags. It performs the following steps when called:

  1. If pattern isundefined, let P be the empty String.
  2. Else, let P be ? ToString(pattern).
  3. If flags isundefined, let F be the empty String.
  4. Else, let F be ? ToString(flags).
  5. If F contains any code unit other than"g","i","m","s","u", or"y"or if it contains the same code unit more than once, throw aSyntaxErrorexception.
  6. If F contains"u", let u betrue; else let u befalse.
  7. If u istrue, then
    1. Let patternText be ! StringToCodePoints(P).
    2. Let patternCharacters be aListwhose elements are the code points of patternText.
  8. Else,
    1. Let patternText be the result of interpreting each of P's 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to the elements.
    2. Let patternCharacters be aListwhose elements are the code unit elements of P.
  9. Let parseResult beParsePattern(patternText, u).
  10. If parseResult is a non-emptyListofSyntaxErrorobjects, throw aSyntaxErrorexception.
  11. Assert: parseResult is aParse NodeforPattern.
  12. Set obj.[[OriginalSource]] to P.
  13. Set obj.[[OriginalFlags]] to F.
  14. Set obj.[[RegExpMatcher]] to theAbstract Closurethat evaluates parseResult by applying the semantics provided in22.2.2using patternCharacters as the pattern'sListofSourceCharactervalues and F as the flag parameters.
  15. Perform ? Set(obj,"lastIndex",+0𝔽,true).
  16. Return obj.

22.2.3.2.3 Static Semantics: ParsePattern ( patternText, u )

The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code points) and u (a Boolean). It performs the following steps when called:

  1. If u istrue, then
    1. Let parseResult beParseText(patternText,Pattern[+U, +N]).
  2. Else,
    1. Let parseResult beParseText(patternText,Pattern[~U, ~N]).
    2. If parseResult is aParse Nodeand parseResult contains aGroupName, then
      1. Set parseResult toParseText(patternText,Pattern[~U, +N]).
  3. Return parseResult.

22.2.3.2.4 RegExpCreate ( P, F )

The abstract operation RegExpCreate takes arguments P and F. It performs the following steps when called:

  1. Let obj be ? RegExpAlloc(%RegExp%).
  2. Return ? RegExpInitialize(obj, P, F).

22.2.3.2.5 EscapeRegExpPattern ( P, F )

The abstract operation EscapeRegExpPattern takes arguments P and F. It performs the following steps when called:

  1. Let S be a String in the form of aPattern[~U](Pattern[+U]if F contains"u") equivalent to P interpreted as UTF-16 encoded Unicode code points (6.1.4), in which certain code points are escaped as described below. S may or may not be identical to P; however, theAbstract Closurethat would result from evaluating S as aPattern[~U](Pattern[+U]if F contains"u") must behave identically to theAbstract Closuregiven by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls to this abstract operation using the same values for P and F must produce identical results.
  2. The code points / or anyLineTerminatoroccurring in the pattern shall be escaped in S as necessary to ensure that thestring-concatenationof"/", S,"/", and F can be parsed (in an appropriate lexical context) as aRegularExpressionLiteralthat behaves identically to the constructed regular expression. For example, if P is"/", then S could be"\/"or"\u002F", among other possibilities, but not"/", because /// followed by F would be parsed as aSingleLineCommentrather than aRegularExpressionLiteral. If P is the empty String, this specification can be met by letting S be"(?:)".
  3. Return S.

22.2.4 Properties of the RegExp Constructor

The RegExpconstructor:

22.2.4.1 RegExp.prototype

The initial value of RegExp.prototype is theRegExp prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

22.2.4.2 get RegExp [ @@species ]

RegExp[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

RegExp prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.

22.2.5 Properties of the RegExp Prototype Object

The RegExp prototype object:

  • is %RegExp.prototype%.
  • is anordinary object.
  • is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
Note

The RegExp prototype object does not have a"valueOf"property of its own; however, it inherits the"valueOf"property from theObject prototype object.

22.2.5.1 RegExp.prototype.constructor

The initial value of RegExp.prototype.constructor is%RegExp%.

22.2.5.2 RegExp.prototype.exec ( string )

Performs a regular expression match of string against the regular expression and returns an Array object containing the results of the match, ornullif string did not match.

The StringToString(string) is searched for an occurrence of the regular expression pattern as follows:

  1. Let R be thethisvalue.
  2. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
  3. Let S be ? ToString(string).
  4. Return ? RegExpBuiltinExec(R, S).

22.2.5.2.1 RegExpExec ( R, S )

The abstract operation RegExpExec takes arguments R and S. It performs the following steps when called:

  1. Assert:Type(R) is Object.
  2. Assert:Type(S) is String.
  3. Let exec be ? Get(R,"exec").
  4. IfIsCallable(exec) istrue, then
    1. Let result be ? Call(exec, R, « S »).
    2. IfType(result) is neither Object nor Null, throw aTypeErrorexception.
    3. Return result.
  5. Perform ? RequireInternalSlot(R, [[RegExpMatcher]]).
  6. Return ? RegExpBuiltinExec(R, S).
Note

If a callable"exec"property is not found this algorithm falls back to attempting to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for prior editions where most built-in algorithms that use regular expressions did not perform a dynamic property lookup of"exec".

22.2.5.2.2 RegExpBuiltinExec ( R, S )

The abstract operation RegExpBuiltinExec takes arguments R and S. It performs the following steps when called:

  1. Assert: R is an initialized RegExp instance.
  2. Assert:Type(S) is String.
  3. Let length be the number of code units in S.
  4. Let lastIndex be(?ToLength(?Get(R,"lastIndex"))).
  5. Let flags be R.[[OriginalFlags]].
  6. If flags contains"g", let global betrue; else let global befalse.
  7. If flags contains"y", let sticky betrue; else let sticky befalse.
  8. If global isfalseand sticky isfalse, set lastIndex to 0.
  9. Let matcher be R.[[RegExpMatcher]].
  10. If flags contains"u", let fullUnicode betrue; else let fullUnicode befalse.
  11. Let matchSucceeded befalse.
  12. Repeat, while matchSucceeded isfalse,
    1. If lastIndex > length, then
      1. If global istrueor sticky istrue, then
        1. Perform ? Set(R,"lastIndex",+0𝔽,true).
      2. Returnnull.
    2. Let r be matcher(S, lastIndex).
    3. If r isfailure, then
      1. If sticky istrue, then
        1. Perform ? Set(R,"lastIndex",+0𝔽,true).
        2. Returnnull.
      2. Set lastIndex toAdvanceStringIndex(S, lastIndex, fullUnicode).
    4. Else,
      1. Assert: r is a State.
      2. Set matchSucceeded totrue.
  13. Let e be r's endIndex value.
  14. If fullUnicode istrue, then
    1. e is an index into the Input character list, derived from S, matched by matcher. Let eUTF be the smallest index into S that corresponds to the character at element e of Input. If e is greater than or equal to the number of elements in Input, then eUTF is the number of code units in S.
    2. Set e to eUTF.
  15. If global istrueor sticky istrue, then
    1. Perform ? Set(R,"lastIndex",𝔽(e),true).
  16. Let n be the number of elements in r's capturesList. (This is the same value as22.2.2.1's NcapturingParens.)
  17. Assert: n < 232 - 1.
  18. Let A be ! ArrayCreate(n + 1).
  19. Assert: Themathematical valueof A's"length"property is n + 1.
  20. Perform ! CreateDataPropertyOrThrow(A,"index",𝔽(lastIndex)).
  21. Perform ! CreateDataPropertyOrThrow(A,"input", S).
  22. Let matchedSubstr be thesubstringof S from lastIndex to e.
  23. Perform ! CreateDataPropertyOrThrow(A,"0", matchedSubstr).
  24. If R contains anyGroupName, then
    1. Let groups be ! OrdinaryObjectCreate(null).
  25. Else,
    1. Let groups beundefined.
  26. Perform ! CreateDataPropertyOrThrow(A,"groups", groups).
  27. For eachintegeri such that i ≥ 1 and in, in ascending order, do
    1. Let captureI be ith element of r's capturesList.
    2. If captureI isundefined, let capturedValue beundefined.
    3. Else if fullUnicode istrue, then
      1. Assert: captureI is aListof code points.
      2. Let capturedValue be ! CodePointsToString(captureI).
    4. Else,
      1. Assert: fullUnicode isfalse.
      2. Assert: captureI is aListof code units.
      3. Let capturedValue be the String value consisting of the code units of captureI.
    5. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(i)), capturedValue).
    6. If the ith capture of R was defined with aGroupName, then
      1. Let s be theCapturingGroupNameof the correspondingRegExpIdentifierName.
      2. Perform ! CreateDataPropertyOrThrow(groups, s, capturedValue).
  28. Return A.

22.2.5.2.3 AdvanceStringIndex ( S, index, unicode )

The abstract operation AdvanceStringIndex takes arguments S (a String), index (a non-negativeinteger), and unicode (a Boolean). It performs the following steps when called:

  1. Assert: index ≤ 253 - 1.
  2. If unicode isfalse, return index + 1.
  3. Let length be the number of code units in S.
  4. If index + 1 ≥ length, return index + 1.
  5. Let cp be ! CodePointAt(S, index).
  6. Return index + cp.[[CodeUnitCount]].

22.2.5.3 get RegExp.prototype.dotAll

RegExp.prototype.dotAll is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
  3. Return ? RegExpHasFlag(R, cu).

22.2.5.3.1 RegExpHasFlag ( R, codeUnit )

The abstract operation RegExpHasFlag takes arguments R (anECMAScript language value) and codeUnit (a code unit). It performs the following steps when called:

  1. IfType(R) is not Object, throw aTypeErrorexception.
  2. If R does not have an [[OriginalFlags]] internal slot, then
    1. IfSameValue(R,%RegExp.prototype%) istrue, returnundefined.
    2. Otherwise, throw aTypeErrorexception.
  3. Let flags be R.[[OriginalFlags]].
  4. If flags contains codeUnit, returntrue.
  5. Returnfalse.

22.2.5.4 get RegExp.prototype.flags

RegExp.prototype.flags is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. IfType(R) is not Object, throw aTypeErrorexception.
  3. Let result be the empty String.
  4. Let global be ! ToBoolean(?Get(R,"global")).
  5. If global istrue, append the code unit 0x0067 (LATIN SMALL LETTER G) as the last code unit of result.
  6. Let ignoreCase be ! ToBoolean(?Get(R,"ignoreCase")).
  7. If ignoreCase istrue, append the code unit 0x0069 (LATIN SMALL LETTER I) as the last code unit of result.
  8. Let multiline be ! ToBoolean(?Get(R,"multiline")).
  9. If multiline istrue, append the code unit 0x006D (LATIN SMALL LETTER M) as the last code unit of result.
  10. Let dotAll be ! ToBoolean(?Get(R,"dotAll")).
  11. If dotAll istrue, append the code unit 0x0073 (LATIN SMALL LETTER S) as the last code unit of result.
  12. Let unicode be ! ToBoolean(?Get(R,"unicode")).
  13. If unicode istrue, append the code unit 0x0075 (LATIN SMALL LETTER U) as the last code unit of result.
  14. Let sticky be ! ToBoolean(?Get(R,"sticky")).
  15. If sticky istrue, append the code unit 0x0079 (LATIN SMALL LETTER Y) as the last code unit of result.
  16. Return result.

22.2.5.5 get RegExp.prototype.global

RegExp.prototype.global is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
  3. Return ? RegExpHasFlag(R, cu).

22.2.5.6 get RegExp.prototype.ignoreCase

RegExp.prototype.ignoreCase is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
  3. Return ? RegExpHasFlag(R, cu).

22.2.5.7 RegExp.prototype [ @@match ] ( string )

When the @@match method is called with argument string, the following steps are taken:

  1. Let rx be thethisvalue.
  2. IfType(rx) is not Object, throw aTypeErrorexception.
  3. Let S be ? ToString(string).
  4. Let global be ! ToBoolean(?Get(rx,"global")).
  5. If global isfalse, then
    1. Return ? RegExpExec(rx, S).
  6. Else,
    1. Assert: global istrue.
    2. Let fullUnicode be ! ToBoolean(?Get(rx,"unicode")).
    3. Perform ? Set(rx,"lastIndex",+0𝔽,true).
    4. Let A be ! ArrayCreate(0).
    5. Let n be 0.
    6. Repeat,
      1. Let result be ? RegExpExec(rx, S).
      2. If result isnull, then
        1. If n = 0, returnnull.
        2. Return A.
      3. Else,
        1. Let matchStr be ? ToString(?Get(result,"0")).
        2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), matchStr).
        3. If matchStr is the empty String, then
          1. Let thisIndex be(?ToLength(?Get(rx,"lastIndex"))).
          2. Let nextIndex beAdvanceStringIndex(S, thisIndex, fullUnicode).
          3. Perform ? Set(rx,"lastIndex",𝔽(nextIndex),true).
        4. Set n to n + 1.

The value of the"name"property of this function is"[Symbol.match]".

Note

The@@matchproperty is used by theIsRegExpabstract operation to identify objects that have the basic behaviour of regular expressions. The absence of a@@matchproperty or the existence of such a property whose value does not Boolean coerce totrueindicates that the object is not intended to be used as a regular expression object.

22.2.5.8 RegExp.prototype [ @@matchAll ] ( string )

When the @@matchAll method is called with argument string, the following steps are taken:

  1. Let R be thethisvalue.
  2. IfType(R) is not Object, throw aTypeErrorexception.
  3. Let S be ? ToString(string).
  4. Let C be ? SpeciesConstructor(R,%RegExp%).
  5. Let flags be ? ToString(?Get(R,"flags")).
  6. Let matcher be ? Construct(C, « R, flags »).
  7. Let lastIndex be ? ToLength(?Get(R,"lastIndex")).
  8. Perform ? Set(matcher,"lastIndex", lastIndex,true).
  9. If flags contains"g", let global betrue.
  10. Else, let global befalse.
  11. If flags contains"u", let fullUnicode betrue.
  12. Else, let fullUnicode befalse.
  13. Return ! CreateRegExpStringIterator(matcher, S, global, fullUnicode).

The value of the"name"property of this function is"[Symbol.matchAll]".

22.2.5.9 get RegExp.prototype.multiline

RegExp.prototype.multiline is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
  3. Return ? RegExpHasFlag(R, cu).

22.2.5.10 RegExp.prototype [ @@replace ] ( string, replaceValue )

When the @@replace method is called with arguments string and replaceValue, the following steps are taken:

  1. Let rx be thethisvalue.
  2. IfType(rx) is not Object, throw aTypeErrorexception.
  3. Let S be ? ToString(string).
  4. Let lengthS be the number of code unit elements in S.
  5. Let functionalReplace beIsCallable(replaceValue).
  6. If functionalReplace isfalse, then
    1. Set replaceValue to ? ToString(replaceValue).
  7. Let global be ! ToBoolean(?Get(rx,"global")).
  8. If global istrue, then
    1. Let fullUnicode be ! ToBoolean(?Get(rx,"unicode")).
    2. Perform ? Set(rx,"lastIndex",+0𝔽,true).
  9. Let results be a new emptyList.
  10. Let done befalse.
  11. Repeat, while done isfalse,
    1. Let result be ? RegExpExec(rx, S).
    2. If result isnull, set done totrue.
    3. Else,
      1. Append result to the end of results.
      2. If global isfalse, set done totrue.
      3. Else,
        1. Let matchStr be ? ToString(?Get(result,"0")).
        2. If matchStr is the empty String, then
          1. Let thisIndex be(?ToLength(?Get(rx,"lastIndex"))).
          2. Let nextIndex beAdvanceStringIndex(S, thisIndex, fullUnicode).
          3. Perform ? Set(rx,"lastIndex",𝔽(nextIndex),true).
  12. Let accumulatedResult be the empty String.
  13. Let nextSourcePosition be 0.
  14. For each element result of results, do
    1. Let resultLength be ? LengthOfArrayLike(result).
    2. Let nCaptures bemax(resultLength - 1, 0).
    3. Let matched be ? ToString(?Get(result,"0")).
    4. Let matchLength be the number of code units in matched.
    5. Let position be ? ToIntegerOrInfinity(?Get(result,"index")).
    6. Set position to the result ofclampingposition between 0 and lengthS.
    7. Let n be 1.
    8. Let captures be a new emptyList.
    9. Repeat, while nnCaptures,
      1. Let capN be ? Get(result, ! ToString(𝔽(n))).
      2. If capN is notundefined, then
        1. Set capN to ? ToString(capN).
      3. Append capN as the last element of captures.
      4. Set n to n + 1.
    10. Let namedCaptures be ? Get(result,"groups").
    11. If functionalReplace istrue, then
      1. Let replacerArgs be « matched ».
      2. Append inListorder the elements of captures to the end of theListreplacerArgs.
      3. Append𝔽(position) and S to replacerArgs.
      4. If namedCaptures is notundefined, then
        1. Append namedCaptures as the last element of replacerArgs.
      5. Let replValue be ? Call(replaceValue,undefined, replacerArgs).
      6. Let replacement be ? ToString(replValue).
    12. Else,
      1. If namedCaptures is notundefined, then
        1. Set namedCaptures to ? ToObject(namedCaptures).
      2. Let replacement be ? GetSubstitution(matched, S, position, captures, namedCaptures, replaceValue).
    13. If positionnextSourcePosition, then
      1. NOTE: position should not normally move backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an access triggered side-effect to change the global flag or other characteristics of rx. In such cases, the corresponding substitution is ignored.
      2. Set accumulatedResult to thestring-concatenationof accumulatedResult, thesubstringof S from nextSourcePosition to position, and replacement.
      3. Set nextSourcePosition to position + matchLength.
  15. If nextSourcePositionlengthS, return accumulatedResult.
  16. Return thestring-concatenationof accumulatedResult and thesubstringof S from nextSourcePosition.

The value of the"name"property of this function is"[Symbol.replace]".

22.2.5.11 RegExp.prototype [ @@search ] ( string )

When the @@search method is called with argument string, the following steps are taken:

  1. Let rx be thethisvalue.
  2. IfType(rx) is not Object, throw aTypeErrorexception.
  3. Let S be ? ToString(string).
  4. Let previousLastIndex be ? Get(rx,"lastIndex").
  5. IfSameValue(previousLastIndex,+0𝔽) isfalse, then
    1. Perform ? Set(rx,"lastIndex",+0𝔽,true).
  6. Let result be ? RegExpExec(rx, S).
  7. Let currentLastIndex be ? Get(rx,"lastIndex").
  8. IfSameValue(currentLastIndex, previousLastIndex) isfalse, then
    1. Perform ? Set(rx,"lastIndex", previousLastIndex,true).
  9. If result isnull, return-1𝔽.
  10. Return ? Get(result,"index").

The value of the"name"property of this function is"[Symbol.search]".

Note

The"lastIndex"and"global"properties of this RegExp object are ignored when performing the search. The"lastIndex"property is left unchanged.

22.2.5.12 get RegExp.prototype.source

RegExp.prototype.source is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. IfType(R) is not Object, throw aTypeErrorexception.
  3. If R does not have an [[OriginalSource]] internal slot, then
    1. IfSameValue(R,%RegExp.prototype%) istrue, return"(?:)".
    2. Otherwise, throw aTypeErrorexception.
  4. Assert: R has an [[OriginalFlags]] internal slot.
  5. Let src be R.[[OriginalSource]].
  6. Let flags be R.[[OriginalFlags]].
  7. ReturnEscapeRegExpPattern(src, flags).

22.2.5.13 RegExp.prototype [ @@split ] ( string, limit )

Note 1

Returns an Array object into which substrings of the result of converting string to a String have been stored. The substrings are determined by searching from left to right for matches of thethisvalue regular expression; these occurrences are not part of any String in the returned array, but serve to divide up the String value.

Thethisvalue may be an empty regular expression or a regular expression that can match an empty String. In this case, the regular expression does not match the emptysubstringat the beginning or end of the input String, nor does it match the emptysubstringat the end of the previous separator match. (For example, if the regular expression matches the empty String, the String is split up into individual code unit elements; the length of the result array equals the length of the String, and eachsubstringcontains one code unit.) Only the first match at a given index of the String is considered, even if backtracking could yield a non-emptysubstringmatch at that index. (For example, /a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while /a*/[Symbol.split]("ab") evaluates to the array ["","b"].)

If string is (or converts to) the empty String, the result depends on whether the regular expression can match the empty String. If it can, the result array contains no elements. Otherwise, the result array contains one element, which is the empty String.

If the regular expression contains capturing parentheses, then each time separator is matched the results (including anyundefinedresults) of the capturing parentheses are spliced into the output array. For example,

/<(\/)?([^<>]+)>/[Symbol.split]("A<B>bold</B>and<CODE>coded</CODE>")

evaluates to the array

["A", undefined, "B", "bold", "/", "B", "and", undefined, "CODE", "coded", "/", "CODE", ""]

If limit is notundefined, then the output array is truncated so that it contains no more than limit elements.

When the @@split method is called, the following steps are taken:

  1. Let rx be thethisvalue.
  2. IfType(rx) is not Object, throw aTypeErrorexception.
  3. Let S be ? ToString(string).
  4. Let C be ? SpeciesConstructor(rx,%RegExp%).
  5. Let flags be ? ToString(?Get(rx,"flags")).
  6. If flags contains"u", let unicodeMatching betrue.
  7. Else, let unicodeMatching befalse.
  8. If flags contains"y", let newFlags be flags.
  9. Else, let newFlags be thestring-concatenationof flags and"y".
  10. Let splitter be ? Construct(C, « rx, newFlags »).
  11. Let A be ! ArrayCreate(0).
  12. Let lengthA be 0.
  13. If limit isundefined, let lim be 232 - 1; else let lim be(?ToUint32(limit)).
  14. If lim is 0, return A.
  15. Let size be the length of S.
  16. If size is 0, then
    1. Let z be ? RegExpExec(splitter, S).
    2. If z is notnull, return A.
    3. Perform ! CreateDataPropertyOrThrow(A,"0", S).
    4. Return A.
  17. Let p be 0.
  18. Let q be p.
  19. Repeat, while q < size,
    1. Perform ? Set(splitter,"lastIndex",𝔽(q),true).
    2. Let z be ? RegExpExec(splitter, S).
    3. If z isnull, set q toAdvanceStringIndex(S, q, unicodeMatching).
    4. Else,
      1. Let e be(?ToLength(?Get(splitter,"lastIndex"))).
      2. Set e tomin(e, size).
      3. If e = p, set q toAdvanceStringIndex(S, q, unicodeMatching).
      4. Else,
        1. Let T be thesubstringof S from p to q.
        2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
        3. Set lengthA to lengthA + 1.
        4. If lengthA = lim, return A.
        5. Set p to e.
        6. Let numberOfCaptures be ? LengthOfArrayLike(z).
        7. Set numberOfCaptures tomax(numberOfCaptures - 1, 0).
        8. Let i be 1.
        9. Repeat, while inumberOfCaptures,
          1. Let nextCapture be ? Get(z, ! ToString(𝔽(i))).
          2. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), nextCapture).
          3. Set i to i + 1.
          4. Set lengthA to lengthA + 1.
          5. If lengthA = lim, return A.
        10. Set q to p.
  20. Let T be thesubstringof S from p to size.
  21. Perform ! CreateDataPropertyOrThrow(A, ! ToString(𝔽(lengthA)), T).
  22. Return A.

The value of the"name"property of this function is"[Symbol.split]".

Note 2

The @@split method ignores the value of the"global"and"sticky"properties of this RegExp object.

22.2.5.14 get RegExp.prototype.sticky

RegExp.prototype.sticky is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
  3. Return ? RegExpHasFlag(R, cu).

22.2.5.15 RegExp.prototype.test ( S )

The following steps are taken:

  1. Let R be thethisvalue.
  2. IfType(R) is not Object, throw aTypeErrorexception.
  3. Let string be ? ToString(S).
  4. Let match be ? RegExpExec(R, string).
  5. If match is notnull, returntrue; else returnfalse.

22.2.5.16 RegExp.prototype.toString ( )

  1. Let R be thethisvalue.
  2. IfType(R) is not Object, throw aTypeErrorexception.
  3. Let pattern be ? ToString(?Get(R,"source")).
  4. Let flags be ? ToString(?Get(R,"flags")).
  5. Let result be thestring-concatenationof"/", pattern,"/", and flags.
  6. Return result.
Note

The returned String has the form of aRegularExpressionLiteralthat evaluates to another RegExp object with the same behaviour as this object.

22.2.5.17 get RegExp.prototype.unicode

RegExp.prototype.unicode is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let R be thethisvalue.
  2. Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
  3. Return ? RegExpHasFlag(R, cu).

22.2.6 Properties of RegExp Instances

RegExp instances are ordinary objects that inherit properties from theRegExp prototype object. RegExp instances have internal slots [[RegExpMatcher]], [[OriginalSource]], and [[OriginalFlags]]. The value of the [[RegExpMatcher]] internal slot is anAbstract Closurerepresentation of thePatternof the RegExp object.

Note

Prior to ECMAScript 2015, RegExp instances were specified as having the own data properties"source","global","ignoreCase", and"multiline". Those properties are now specified as accessor properties of RegExp.prototype.

RegExp instances also have the following property:

22.2.6.1 lastIndex

The value of the"lastIndex"property specifies the String index at which to start the next match. It is coerced to anintegral Numberwhen used (see22.2.5.2.2). This property shall have the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.

22.2.7 RegExp String Iterator Objects

A RegExp String Iterator is an object, that represents a specific iteration over some specific String instance object, matching against some specific RegExp instance object. There is not a namedconstructorfor RegExp String Iterator objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance objects.

22.2.7.1 CreateRegExpStringIterator ( R, S, global, fullUnicode )

The abstract operation CreateRegExpStringIterator takes arguments R, S, global, and fullUnicode. It performs the following steps when called:

  1. Assert:Type(S) is String.
  2. Assert:Type(global) is Boolean.
  3. Assert:Type(fullUnicode) is Boolean.
  4. Let closure be a newAbstract Closurewith no parameters that captures R, S, global, and fullUnicode and performs the following steps when called:
    1. Repeat,
      1. Let match be ? RegExpExec(R, S).
      2. If match isnull, returnundefined.
      3. If global isfalse, then
        1. Perform ? Yield(match).
        2. Returnundefined.
      4. Let matchStr be ? ToString(?Get(match,"0")).
      5. If matchStr is the empty String, then
        1. Let thisIndex be(?ToLength(?Get(R,"lastIndex"))).
        2. Let nextIndex be ! AdvanceStringIndex(S, thisIndex, fullUnicode).
        3. Perform ? Set(R,"lastIndex",𝔽(nextIndex),true).
      6. Perform ? Yield(match).
  5. Return ! CreateIteratorFromClosure(closure,"%RegExpStringIteratorPrototype%",%RegExpStringIteratorPrototype%).

22.2.7.2 The %RegExpStringIteratorPrototype% Object

The %RegExpStringIteratorPrototype% object:

  • has properties that are inherited by all RegExp String Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has the following properties:

22.2.7.2.1 %RegExpStringIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(thisvalue,empty,"%RegExpStringIteratorPrototype%").

22.2.7.2.2 %RegExpStringIteratorPrototype% [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"RegExp String Iterator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

23 Indexed Collections

23.1 Array Objects

Array objects are exotic objects that give special treatment to a certain class of property names. See10.4.2for a definition of this special treatment.

23.1.1 The Array Constructor

The Arrayconstructor:

  • is %Array%.
  • is the initial value of the"Array"property of theglobal object.
  • creates and initializes a newArray exotic objectwhen called as aconstructor.
  • also creates and initializes a new Array object when called as a function rather than as aconstructor. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.
  • is a function whose behaviour differs based upon the number and types of its arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Arrayconstructorto initialize subclass instances that are Array exotic objects. However, most of the Array.prototype methods are generic methods that are not dependent upon theirthisvalue being anArray exotic object.
  • has a"length"property whose value is1𝔽.

23.1.1.1 Array ( ...values )

When the Array function is called, the following steps are taken:

  1. If NewTarget isundefined, let newTarget be theactive function object; else let newTarget be NewTarget.
  2. Let proto be ? GetPrototypeFromConstructor(newTarget,"%Array.prototype%").
  3. Let numberOfArgs be the number of elements in values.
  4. If numberOfArgs = 0, then
    1. Return ! ArrayCreate(0, proto).
  5. Else if numberOfArgs = 1, then
    1. Let len be values[0].
    2. Let array be ! ArrayCreate(0, proto).
    3. IfType(len) is not Number, then
      1. Perform ! CreateDataPropertyOrThrow(array,"0", len).
      2. Let intLen be1𝔽.
    4. Else,
      1. Let intLen be ! ToUint32(len).
      2. IfSameValueZero(intLen, len) isfalse, throw aRangeErrorexception.
    5. Perform ! Set(array,"length", intLen,true).
    6. Return array.
  6. Else,
    1. Assert: numberOfArgs ≥ 2.
    2. Let array be ? ArrayCreate(numberOfArgs, proto).
    3. Let k be 0.
    4. Repeat, while k < numberOfArgs,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Let itemK be values[k].
      3. Perform ! CreateDataPropertyOrThrow(array, Pk, itemK).
      4. Set k to k + 1.
    5. Assert: Themathematical valueof array's"length"property is numberOfArgs.
    6. Return array.

23.1.2 Properties of the Array Constructor

The Arrayconstructor:

23.1.2.1 Array.from ( items [ , mapfn [ , thisArg ] ] )

When the from method is called, the following steps are taken:

  1. Let C be thethisvalue.
  2. If mapfn isundefined, let mapping befalse.
  3. Else,
    1. IfIsCallable(mapfn) isfalse, throw aTypeErrorexception.
    2. Let mapping betrue.
  4. Let usingIterator be ? GetMethod(items,@@iterator).
  5. If usingIterator is notundefined, then
    1. IfIsConstructor(C) istrue, then
      1. Let A be ? Construct(C).
    2. Else,
      1. Let A be ! ArrayCreate(0).
    3. Let iteratorRecord be ? GetIterator(items,sync, usingIterator).
    4. Let k be 0.
    5. Repeat,
      1. If k ≥ 253 - 1, then
        1. Let error beThrowCompletion(a newly createdTypeErrorobject).
        2. Return ? IteratorClose(iteratorRecord, error).
      2. Let Pk be ! ToString(𝔽(k)).
      3. Let next be ? IteratorStep(iteratorRecord).
      4. If next isfalse, then
        1. Perform ? Set(A,"length",𝔽(k),true).
        2. Return A.
      5. Let nextValue be ? IteratorValue(next).
      6. If mapping istrue, then
        1. Let mappedValue beCall(mapfn, thisArg, « nextValue,𝔽(k) »).
        2. If mappedValue is anabrupt completion, return ? IteratorClose(iteratorRecord, mappedValue).
        3. Set mappedValue to mappedValue.[[Value]].
      7. Else, let mappedValue be nextValue.
      8. Let defineStatus beCreateDataPropertyOrThrow(A, Pk, mappedValue).
      9. If defineStatus is anabrupt completion, return ? IteratorClose(iteratorRecord, defineStatus).
      10. Set k to k + 1.
  6. NOTE: items is not an Iterable so assume it is anarray-like object.
  7. Let arrayLike be ! ToObject(items).
  8. Let len be ? LengthOfArrayLike(arrayLike).
  9. IfIsConstructor(C) istrue, then
    1. Let A be ? Construct(C, «𝔽(len) »).
  10. Else,
    1. Let A be ? ArrayCreate(len).
  11. Let k be 0.
  12. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. If mapping istrue, then
      1. Let mappedValue be ? Call(mapfn, thisArg, « kValue,𝔽(k) »).
    4. Else, let mappedValue be kValue.
    5. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
    6. Set k to k + 1.
  13. Perform ? Set(A,"length",𝔽(len),true).
  14. Return A.
Note

The from function is an intentionally generic factory method; it does not require that itsthisvalue be the Arrayconstructor. Therefore it can be transferred to or inherited by any other constructors that may be called with a single numeric argument.

23.1.2.2 Array.isArray ( arg )

When the isArray method is called, the following steps are taken:

  1. Return ? IsArray(arg).

23.1.2.3 Array.of ( ...items )

When the of method is called, the following steps are taken:

  1. Let len be the number of elements in items.
  2. Let lenNumber be𝔽(len).
  3. Let C be thethisvalue.
  4. IfIsConstructor(C) istrue, then
    1. Let A be ? Construct(C, « lenNumber »).
  5. Else,
    1. Let A be ? ArrayCreate(len).
  6. Let k be 0.
  7. Repeat, while k < len,
    1. Let kValue be items[k].
    2. Let Pk be ! ToString(𝔽(k)).
    3. Perform ? CreateDataPropertyOrThrow(A, Pk, kValue).
    4. Set k to k + 1.
  8. Perform ? Set(A,"length", lenNumber,true).
  9. Return A.
Note

The of function is an intentionally generic factory method; it does not require that itsthisvalue be the Arrayconstructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.

23.1.2.4 Array.prototype

The value of Array.prototype is theArray prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

23.1.2.5 get Array [ @@species ]

Array[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

Array prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.

23.1.3 Properties of the Array Prototype Object

The Array prototype object:

  • is %Array.prototype%.
  • is anArray exotic objectand has the internal methods specified for such objects.
  • has a"length"property whose initial value is+0𝔽 and whose attributes are { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
Note

The Array prototype object is specified to be anArray exotic objectto ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.

23.1.3.1 Array.prototype.concat ( ...items )

Returns an array containing the array elements of the object followed by the array elements of each argument.

When the concat method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let A be ? ArraySpeciesCreate(O, 0).
  3. Let n be 0.
  4. Prepend O to items.
  5. For each element E of items, do
    1. Let spreadable be ? IsConcatSpreadable(E).
    2. If spreadable istrue, then
      1. Let k be 0.
      2. Let len be ? LengthOfArrayLike(E).
      3. If n + len > 253 - 1, throw aTypeErrorexception.
      4. Repeat, while k < len,
        1. Let P be ! ToString(𝔽(k)).
        2. Let exists be ? HasProperty(E, P).
        3. If exists istrue, then
          1. Let subElement be ? Get(E, P).
          2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), subElement).
        4. Set n to n + 1.
        5. Set k to k + 1.
    3. Else,
      1. NOTE: E is added as a single item rather than spread.
      2. If n ≥ 253 - 1, throw aTypeErrorexception.
      3. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), E).
      4. Set n to n + 1.
  6. Perform ? Set(A,"length",𝔽(n),true).
  7. Return A.

The"length"property of the concat method is1𝔽.

Note 1

The explicit setting of the"length"property in step6is necessary to ensure that its value is correct in situations where the trailing elements of the result Array are not present.

Note 2

The concat function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.1.1 IsConcatSpreadable ( O )

The abstract operation IsConcatSpreadable takes argument O. It performs the following steps when called:

  1. IfType(O) is not Object, returnfalse.
  2. Let spreadable be ? Get(O,@@isConcatSpreadable).
  3. If spreadable is notundefined, return ! ToBoolean(spreadable).
  4. Return ? IsArray(O).

23.1.3.2 Array.prototype.constructor

The initial value of Array.prototype.constructor is%Array%.

23.1.3.3 Array.prototype.copyWithin ( target, start [ , end ] )

Note 1

The end argument is optional. If it is not provided, the length of thethisvalue is used.

Note 2

If target is negative, it is treated aslength + targetwhere length is the length of the array. If start is negative, it is treated aslength + start. If end is negative, it is treated aslength + end.

When the copyWithin method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeTarget be ? ToIntegerOrInfinity(target).
  4. If relativeTarget is -∞, let to be 0.
  5. Else if relativeTarget < 0, let to bemax(len + relativeTarget, 0).
  6. Else, let to bemin(relativeTarget, len).
  7. Let relativeStart be ? ToIntegerOrInfinity(start).
  8. If relativeStart is -∞, let from be 0.
  9. Else if relativeStart < 0, let from bemax(len + relativeStart, 0).
  10. Else, let from bemin(relativeStart, len).
  11. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  12. If relativeEnd is -∞, let final be 0.
  13. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  14. Else, let final bemin(relativeEnd, len).
  15. Let count bemin(final - from, len - to).
  16. If from < to and to < from + count, then
    1. Let direction be -1.
    2. Set from to from + count - 1.
    3. Set to to to + count - 1.
  17. Else,
    1. Let direction be 1.
  18. Repeat, while count > 0,
    1. Let fromKey be ! ToString(𝔽(from)).
    2. Let toKey be ! ToString(𝔽(to)).
    3. Let fromPresent be ? HasProperty(O, fromKey).
    4. If fromPresent istrue, then
      1. Let fromVal be ? Get(O, fromKey).
      2. Perform ? Set(O, toKey, fromVal,true).
    5. Else,
      1. Assert: fromPresent isfalse.
      2. Perform ? DeletePropertyOrThrow(O, toKey).
    6. Set from to from + direction.
    7. Set to to to + direction.
    8. Set count to count - 1.
  19. Return O.
Note 3

The copyWithin function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.4 Array.prototype.entries ( )

When the entries method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. ReturnCreateArrayIterator(O,key+value).

23.1.3.5 Array.prototype.every ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. every calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returnsfalse. If such an element is found, every immediately returnsfalse. Otherwise, if callbackfn returnedtruefor all elements, every will returntrue. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

every does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by every is set before the first call to callbackfn. Elements which are appended to the array after the call to every begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time every visits them; elements that are deleted after the call to every begins and before being visited are not visited. every acts like the "for all" quantifier in mathematics. In particular, for an empty array, it returnstrue.

When the every method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
      3. If testResult isfalse, returnfalse.
    4. Set k to k + 1.
  6. Returntrue.
Note 2

The every function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.6 Array.prototype.fill ( value [ , start [ , end ] ] )

Note 1

The start argument is optional. If it is not provided,+0𝔽 is used.

The end argument is optional. If it is not provided, the length of thethisvalue is used.

Note 2

If start is negative, it is treated aslength + startwhere length is the length of the array. If end is negative, it is treated aslength + end.

When the fill method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart is -∞, let k be 0.
  5. Else if relativeStart < 0, let k bemax(len + relativeStart, 0).
  6. Else, let k bemin(relativeStart, len).
  7. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  8. If relativeEnd is -∞, let final be 0.
  9. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  10. Else, let final bemin(relativeEnd, len).
  11. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Perform ? Set(O, Pk, value,true).
    3. Set k to k + 1.
  12. Return O.
Note 3

The fill function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.7 Array.prototype.filter ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. filter calls callbackfn once for each element in the array, in ascending order, and constructs a new array of all the values for which callbackfn returnstrue. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

filter does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by filter is set before the first call to callbackfn. Elements which are appended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of the array are changed their value as passed to callbackfn will be the value at the time filter visits them; elements that are deleted after the call to filter begins and before being visited are not visited.

When the filter method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let A be ? ArraySpeciesCreate(O, 0).
  5. Let k be 0.
  6. Let to be 0.
  7. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Let selected be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
      3. If selected istrue, then
        1. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(to)), kValue).
        2. Set to to to + 1.
    4. Set k to k + 1.
  8. Return A.
Note 2

The filter function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.8 Array.prototype.find ( predicate [ , thisArg ] )

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. find calls predicate once for each element of the array, in ascending order, until it finds one where predicate returnstrue. If such an element is found, find immediately returns that element value. Otherwise, find returnsundefined.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of predicate. If it is not provided,undefinedis used instead.

predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.

find does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.

The range of elements processed by find is set before the first call to predicate. Elements that are appended to the array after the call to find begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that find visits them; elements that are deleted after the call to find begins and before being visited are not visited.

When the find method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(predicate) isfalse, throw aTypeErrorexception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(predicate, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult istrue, return kValue.
    5. Set k to k + 1.
  6. Returnundefined.
Note 2

The find function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.9 Array.prototype.findIndex ( predicate [ , thisArg ] )

Note 1

predicate should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. findIndex calls predicate once for each element of the array, in ascending order, until it finds one where predicate returnstrue. If such an element is found, findIndex immediately returns the index of that element value. Otherwise, findIndex returns -1.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of predicate. If it is not provided,undefinedis used instead.

predicate is called with three arguments: the value of the element, the index of the element, and the object being traversed.

findIndex does not directly mutate the object on which it is called but the object may be mutated by the calls to predicate.

The range of elements processed by findIndex is set before the first call to predicate. Elements that are appended to the array after the call to findIndex begins will not be visited by predicate. If existing elements of the array are changed, their value as passed to predicate will be the value at the time that findIndex visits them; elements that are deleted after the call to findIndex begins and before being visited are not visited.

When the findIndex method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(predicate) isfalse, throw aTypeErrorexception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(predicate, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult istrue, return𝔽(k).
    5. Set k to k + 1.
  6. Return-1𝔽.
Note 2

The findIndex function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.10 Array.prototype.flat ( [ depth ] )

When the flat method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let sourceLen be ? LengthOfArrayLike(O).
  3. Let depthNum be 1.
  4. If depth is notundefined, then
    1. Set depthNum to ? ToIntegerOrInfinity(depth).
    2. If depthNum < 0, set depthNum to 0.
  5. Let A be ? ArraySpeciesCreate(O, 0).
  6. Perform ? FlattenIntoArray(A, O, sourceLen, 0, depthNum).
  7. Return A.

23.1.3.10.1 FlattenIntoArray ( target, source, sourceLen, start, depth [ , mapperFunction [ , thisArg ] ] )

The abstract operation FlattenIntoArray takes arguments target, source, sourceLen (a non-negativeinteger), start (a non-negativeinteger), and depth (a non-negativeintegeror +∞) and optional arguments mapperFunction and thisArg. It performs the following steps when called:

  1. Assert:Type(target) is Object.
  2. Assert:Type(source) is Object.
  3. Assert: If mapperFunction is present, then ! IsCallable(mapperFunction) istrue, thisArg is present, and depth is 1.
  4. Let targetIndex be start.
  5. Let sourceIndex be+0𝔽.
  6. Repeat, while(sourceIndex) < sourceLen,
    1. Let P be ! ToString(sourceIndex).
    2. Let exists be ? HasProperty(source, P).
    3. If exists istrue, then
      1. Let element be ? Get(source, P).
      2. If mapperFunction is present, then
        1. Set element to ? Call(mapperFunction, thisArg, « element, sourceIndex, source »).
      3. Let shouldFlatten befalse.
      4. If depth > 0, then
        1. Set shouldFlatten to ? IsArray(element).
      5. If shouldFlatten istrue, then
        1. If depth is +∞, let newDepth be +∞.
        2. Else, let newDepth be depth - 1.
        3. Let elementLen be ? LengthOfArrayLike(element).
        4. Set targetIndex to ? FlattenIntoArray(target, element, elementLen, targetIndex, newDepth).
      6. Else,
        1. If targetIndex ≥ 253 - 1, throw aTypeErrorexception.
        2. Perform ? CreateDataPropertyOrThrow(target, ! ToString(𝔽(targetIndex)), element).
        3. Set targetIndex to targetIndex + 1.
    4. Set sourceIndex to sourceIndex +1𝔽.
  7. Return targetIndex.

23.1.3.11 Array.prototype.flatMap ( mapperFunction [ , thisArg ] )

When the flatMap method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let sourceLen be ? LengthOfArrayLike(O).
  3. If ! IsCallable(mapperFunction) isfalse, throw aTypeErrorexception.
  4. Let A be ? ArraySpeciesCreate(O, 0).
  5. Perform ? FlattenIntoArray(A, O, sourceLen, 0, 1, mapperFunction, thisArg).
  6. Return A.

23.1.3.12 Array.prototype.forEach ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by forEach is set before the first call to callbackfn. Elements which are appended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time forEach visits them; elements that are deleted after the call to forEach begins and before being visited are not visited.

When the forEach method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Perform ? Call(callbackfn, thisArg, « kValue,𝔽(k), O »).
    4. Set k to k + 1.
  6. Returnundefined.
Note 2

The forEach function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.13 Array.prototype.includes ( searchElement [ , fromIndex ] )

Note 1

includes compares searchElement to the elements of the array, in ascending order, using theSameValueZeroalgorithm, and if found at any position, returnstrue; otherwise,falseis returned.

The optional second argument fromIndex defaults to+0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array,falseis returned, i.e. the array will not be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽, the whole array will be searched.

When the includes method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len is 0, returnfalse.
  4. Let n be ? ToIntegerOrInfinity(fromIndex).
  5. Assert: If fromIndex isundefined, then n is 0.
  6. If n is +∞, returnfalse.
  7. Else if n is -∞, set n to 0.
  8. If n ≥ 0, then
    1. Let k be n.
  9. Else,
    1. Let k be len + n.
    2. If k < 0, set k to 0.
  10. Repeat, while k < len,
    1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
    2. IfSameValueZero(searchElement, elementK) istrue, returntrue.
    3. Set k to k + 1.
  11. Returnfalse.
Note 2

The includes function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

Note 3

The includes method intentionally differs from the similar indexOf method in two ways. First, it uses theSameValueZeroalgorithm, instead ofIsStrictlyEqual, allowing it to detectNaNarray elements. Second, it does not skip missing array elements, instead treating them asundefined.

23.1.3.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )

indexOf compares searchElement to the elements of the array, in ascending order, using theIsStrictlyEqualalgorithm, and if found at one or more indices, returns the smallest such index; otherwise,-1𝔽 is returned.

Note 1

The optional second argument fromIndex defaults to+0𝔽 (i.e. the whole array is searched). If it is greater than or equal to the length of the array,-1𝔽 is returned, i.e. the array will not be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽, the whole array will be searched.

When the indexOf method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len is 0, return-1𝔽.
  4. Let n be ? ToIntegerOrInfinity(fromIndex).
  5. Assert: If fromIndex isundefined, then n is 0.
  6. If n is +∞, return-1𝔽.
  7. Else if n is -∞, set n to 0.
  8. If n ≥ 0, then
    1. Let k be n.
  9. Else,
    1. Let k be len + n.
    2. If k < 0, set k to 0.
  10. Repeat, while k < len,
    1. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent istrue, then
      1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
      2. Let same beIsStrictlyEqual(searchElement, elementK).
      3. If same istrue, return𝔽(k).
    3. Set k to k + 1.
  11. Return-1𝔽.
Note 2

The indexOf function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.15 Array.prototype.join ( separator )

The elements of the array are converted to Strings, and these Strings are then concatenated, separated by occurrences of the separator. If no separator is provided, a single comma is used as the separator.

When the join method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If separator isundefined, let sep be the single-element String",".
  4. Else, let sep be ? ToString(separator).
  5. Let R be the empty String.
  6. Let k be 0.
  7. Repeat, while k < len,
    1. If k > 0, set R to thestring-concatenationof R and sep.
    2. Let element be ? Get(O, ! ToString(𝔽(k))).
    3. If element isundefinedornull, let next be the empty String; otherwise, let next be ? ToString(element).
    4. Set R to thestring-concatenationof R and next.
    5. Set k to k + 1.
  8. Return R.
Note

The join function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.16 Array.prototype.keys ( )

When the keys method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. ReturnCreateArrayIterator(O,key).

23.1.3.17 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

Note 1

lastIndexOf compares searchElement to the elements of the array in descending order using theIsStrictlyEqualalgorithm, and if found at one or more indices, returns the largest such index; otherwise,-1𝔽 is returned.

The optional second argument fromIndex defaults to the array's length minus one (i.e. the whole array is searched). If it is greater than or equal to the length of the array, the whole array will be searched. If it is less than+0𝔽, it is used as the offset from the end of the array to compute fromIndex. If the computed index is less than+0𝔽,-1𝔽 is returned.

When the lastIndexOf method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len is 0, return-1𝔽.
  4. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex); else let n be len - 1.
  5. If n is -∞, return-1𝔽.
  6. If n ≥ 0, then
    1. Let k bemin(n, len - 1).
  7. Else,
    1. Let k be len + n.
  8. Repeat, while k ≥ 0,
    1. Let kPresent be ? HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent istrue, then
      1. Let elementK be ? Get(O, ! ToString(𝔽(k))).
      2. Let same beIsStrictlyEqual(searchElement, elementK).
      3. If same istrue, return𝔽(k).
    3. Set k to k - 1.
  9. Return-1𝔽.
Note 2

The lastIndexOf function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.18 Array.prototype.map ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in the array, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

map does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by map is set before the first call to callbackfn. Elements which are appended to the array after the call to map begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time map visits them; elements that are deleted after the call to map begins and before being visited are not visited.

When the map method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let A be ? ArraySpeciesCreate(O, len).
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Let mappedValue be ? Call(callbackfn, thisArg, « kValue,𝔽(k), O »).
      3. Perform ? CreateDataPropertyOrThrow(A, Pk, mappedValue).
    4. Set k to k + 1.
  7. Return A.
Note 2

The map function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.19 Array.prototype.pop ( )

Note 1

The last element of the array is removed from the array and returned.

When the pop method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, then
    1. Perform ? Set(O,"length",+0𝔽,true).
    2. Returnundefined.
  4. Else,
    1. Assert: len > 0.
    2. Let newLen be𝔽(len - 1).
    3. Let index be ! ToString(newLen).
    4. Let element be ? Get(O, index).
    5. Perform ? DeletePropertyOrThrow(O, index).
    6. Perform ? Set(O,"length", newLen,true).
    7. Return element.
Note 2

The pop function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.20 Array.prototype.push ( ...items )

Note 1

The arguments are appended to the end of the array, in the order in which they appear. The new length of the array is returned as the result of the call.

When the push method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let argCount be the number of elements in items.
  4. If len + argCount > 253 - 1, throw aTypeErrorexception.
  5. For each element E of items, do
    1. Perform ? Set(O, ! ToString(𝔽(len)), E,true).
    2. Set len to len + 1.
  6. Perform ? Set(O,"length",𝔽(len),true).
  7. Return𝔽(len).

The"length"property of the push method is1𝔽.

Note 2

The push function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )

Note 1

callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once for each element after the first element present in the array, in ascending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time that callback is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal to the first value in the array. If no initialValue was supplied, then previousValue will be equal to the first value in the array and currentValue will be equal to the second. It is aTypeErrorif the array contains no elements and initialValue is not provided.

reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduce is set before the first call to callbackfn. Elements that are appended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them; elements that are deleted after the call to reduce begins and before being visited are not visited.

When the reduce method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. If len = 0 and initialValue is not present, throw aTypeErrorexception.
  5. Let k be 0.
  6. Let accumulator beundefined.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else,
    1. Let kPresent befalse.
    2. Repeat, while kPresent isfalseand k < len,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Set kPresent to ? HasProperty(O, Pk).
      3. If kPresent istrue, then
        1. Set accumulator to ? Get(O, Pk).
      4. Set k to k + 1.
    3. If kPresent isfalse, throw aTypeErrorexception.
  9. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
    4. Set k to k + 1.
  10. Return accumulator.
Note 2

The reduce function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )

Note 1

callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function, once for each element after the first element present in the array, in descending order.

callbackfn is called with four arguments: the previousValue (value from the previous call to callbackfn), the currentValue (value of the current element), the currentIndex, and the object being traversed. The first time the function is called, the previousValue and currentValue can be one of two values. If an initialValue was supplied in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will be equal to the last value in the array. If no initialValue was supplied, then previousValue will be equal to the last value in the array and currentValue will be equal to the second-to-last value. It is aTypeErrorif the array contains no elements and initialValue is not provided.

reduceRight does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that are appended to the array after the call to reduceRight begins will not be visited by callbackfn. If existing elements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at the time reduceRight visits them; elements that are deleted after the call to reduceRight begins and before being visited are not visited.

When the reduceRight method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. If len is 0 and initialValue is not present, throw aTypeErrorexception.
  5. Let k be len - 1.
  6. Let accumulator beundefined.
  7. If initialValue is present, then
    1. Set accumulator to initialValue.
  8. Else,
    1. Let kPresent befalse.
    2. Repeat, while kPresent isfalseand k ≥ 0,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Set kPresent to ? HasProperty(O, Pk).
      3. If kPresent istrue, then
        1. Set accumulator to ? Get(O, Pk).
      4. Set k to k - 1.
    3. If kPresent isfalse, throw aTypeErrorexception.
  9. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
    4. Set k to k - 1.
  10. Return accumulator.
Note 2

The reduceRight function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.23 Array.prototype.reverse ( )

Note 1

The elements of the array are rearranged so as to reverse their order. The object is returned as the result of the call.

When the reverse method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let middle befloor(len / 2).
  4. Let lower be 0.
  5. Repeat, while lowermiddle,
    1. Let upper be len - lower - 1.
    2. Let upperP be ! ToString(𝔽(upper)).
    3. Let lowerP be ! ToString(𝔽(lower)).
    4. Let lowerExists be ? HasProperty(O, lowerP).
    5. If lowerExists istrue, then
      1. Let lowerValue be ? Get(O, lowerP).
    6. Let upperExists be ? HasProperty(O, upperP).
    7. If upperExists istrue, then
      1. Let upperValue be ? Get(O, upperP).
    8. If lowerExists istrueand upperExists istrue, then
      1. Perform ? Set(O, lowerP, upperValue,true).
      2. Perform ? Set(O, upperP, lowerValue,true).
    9. Else if lowerExists isfalseand upperExists istrue, then
      1. Perform ? Set(O, lowerP, upperValue,true).
      2. Perform ? DeletePropertyOrThrow(O, upperP).
    10. Else if lowerExists istrueand upperExists isfalse, then
      1. Perform ? DeletePropertyOrThrow(O, lowerP).
      2. Perform ? Set(O, upperP, lowerValue,true).
    11. Else,
      1. Assert: lowerExists and upperExists are bothfalse.
      2. No action is required.
    12. Set lower to lower + 1.
  6. Return O.
Note 2

The reverse function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.24 Array.prototype.shift ( )

The first element of the array is removed from the array and returned.

When the shift method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. If len = 0, then
    1. Perform ? Set(O,"length",+0𝔽,true).
    2. Returnundefined.
  4. Let first be ? Get(O,"0").
  5. Let k be 1.
  6. Repeat, while k < len,
    1. Let from be ! ToString(𝔽(k)).
    2. Let to be ! ToString(𝔽(k - 1)).
    3. Let fromPresent be ? HasProperty(O, from).
    4. If fromPresent istrue, then
      1. Let fromVal be ? Get(O, from).
      2. Perform ? Set(O, to, fromVal,true).
    5. Else,
      1. Assert: fromPresent isfalse.
      2. Perform ? DeletePropertyOrThrow(O, to).
    6. Set k to k + 1.
  7. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(len - 1))).
  8. Perform ? Set(O,"length",𝔽(len - 1),true).
  9. Return first.
Note

The shift function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.25 Array.prototype.slice ( start, end )

The slice method returns an array containing the elements of the array from element start up to, but not including, element end (or through the end of the array if end isundefined). If start is negative, it is treated aslength + startwhere length is the length of the array. If end is negative, it is treated aslength + endwhere length is the length of the array.

When the slice method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart is -∞, let k be 0.
  5. Else if relativeStart < 0, let k bemax(len + relativeStart, 0).
  6. Else, let k bemin(relativeStart, len).
  7. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  8. If relativeEnd is -∞, let final be 0.
  9. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  10. Else, let final bemin(relativeEnd, len).
  11. Let count bemax(final - k, 0).
  12. Let A be ? ArraySpeciesCreate(O, count).
  13. Let n be 0.
  14. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(n)), kValue).
    4. Set k to k + 1.
    5. Set n to n + 1.
  15. Perform ? Set(A,"length",𝔽(n),true).
  16. Return A.
Note 1

The explicit setting of the"length"property of the result Array in step15was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting"length"became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

Note 2

The slice function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.26 Array.prototype.some ( callbackfn [ , thisArg ] )

Note 1

callbackfn should be a function that accepts three arguments and returns a value that is coercible to a Boolean value. some calls callbackfn once for each element present in the array, in ascending order, until it finds one where callbackfn returnstrue. If such an element is found, some immediately returnstrue. Otherwise, some returnsfalse. callbackfn is called only for elements of the array which actually exist; it is not called for missing elements of the array.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the element, the index of the element, and the object being traversed.

some does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

The range of elements processed by some is set before the first call to callbackfn. Elements that are appended to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array are changed, their value as passed to callbackfn will be the value at the time that some visits them; elements that are deleted after the call to some begins and before being visited are not visited. some acts like the "exists" quantifier in mathematics. In particular, for an empty array, it returnsfalse.

When the some method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(O, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(O, Pk).
      2. Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
      3. If testResult istrue, returntrue.
    4. Set k to k + 1.
  6. Returnfalse.
Note 2

The some function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.27 Array.prototype.sort ( comparefn )

The elements of this array are sorted. The sort must be stable (that is, elements that compare equal must remain in their original order). If comparefn is notundefined, it should be a function that accepts two arguments x and y and returns a negative Number if x < y, a positive Number if x > y, or a zero otherwise.

When the sort method is called, the following steps are taken:

  1. If comparefn is notundefinedandIsCallable(comparefn) isfalse, throw aTypeErrorexception.
  2. Let obj be ? ToObject(thisvalue).
  3. Let len be ? LengthOfArrayLike(obj).
  4. Let items be a new emptyList.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kPresent be ? HasProperty(obj, Pk).
    3. If kPresent istrue, then
      1. Let kValue be ? Get(obj, Pk).
      2. Append kValue to items.
    4. Set k to k + 1.
  7. Let itemCount be the number of elements in items.
  8. Sort items using animplementation-definedsequence of calls toSortCompare. If any such call returns anabrupt completion, stop before performing any further calls toSortCompareor steps in this algorithm and return that completion.
  9. Let j be 0.
  10. Repeat, while j < itemCount,
    1. Perform ? Set(obj, ! ToString(𝔽(j)), items[j],true).
    2. Set j to j + 1.
  11. Repeat, while j < len,
    1. Perform ? DeletePropertyOrThrow(obj, ! ToString(𝔽(j))).
    2. Set j to j + 1.
  12. Return obj.

The sort order is the ordering, after completion of this function, of theinteger-indexedproperty values of obj whoseintegerindexes are less than len. The result of the sort function is then determined as follows:

The sort order isimplementation-definedif any of the following conditions is true:

  • If comparefn is notundefinedand is not a consistent comparison function for the elements of items (see below).
  • If comparefn isundefinedandSortComparedoes not act as a consistent comparison function.
  • If comparefn isundefinedand all applications ofToString, to any specific value passed as an argument toSortCompare, do not produce the same result.

Unless the sort order is specified above to beimplementation-defined, items must satisfy all of the following conditions after executing step8of the algorithm above:

  • There must be some mathematical permutation π of the non-negative integers less than itemCount, such that for every non-negativeintegerj less than itemCount, the elementold[j]is exactly the same asnew[π(j)].
  • Then for all non-negative integers j and k, each less than itemCount, ifSortCompare(old[j], old[k]) < 0(seeSortComparebelow), thenπ(j) < π(k).

Here the notationold[j]is used to refer toitems[j]before step8is executed, and the notationnew[j]to refer toitems[j]after step8has been executed.

A function comparefn is a consistent comparison function for a set of values S if all of the requirements below are met for all values a, b, and c (possibly the same value) in the set S: The notationa <CF bmeanscomparefn(a, b) < 0;a =CF bmeanscomparefn(a, b) = 0(of either sign); anda >CF bmeanscomparefn(a, b) > 0.

  • Calling comparefn(a, b) always returns the same value v when given a specific pair of values a and b as its two arguments. Furthermore,Type(v) is Number, and v is notNaN. Note that this implies that exactly one of a <CF b, a =CF b, and a >CF b will be true for a given pair of a and b.
  • Calling comparefn(a, b) does not modify obj or any object on obj's prototype chain.
  • a =CF a (reflexivity)
  • If a =CF b, then b =CF a (symmetry)
  • If a =CF b and b =CF c, then a =CF c (transitivity of =CF)
  • If a <CF b and b <CF c, then a <CF c (transitivity of <CF)
  • If a >CF b and b >CF c, then a >CF c (transitivity of >CF)
Note 1

The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalence classes and that these equivalence classes are totally ordered.

Note 2

The sort function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore, it can be transferred to other kinds of objects for use as a method.

23.1.3.27.1 SortCompare ( x, y )

The abstract operation SortCompare takes arguments x and y. It also has access to the comparefn argument passed to the current invocation of the sort method. It performs the following steps when called:

  1. If x and y are bothundefined, return+0𝔽.
  2. If x isundefined, return1𝔽.
  3. If y isundefined, return-1𝔽.
  4. If comparefn is notundefined, then
    1. Let v be ? ToNumber(?Call(comparefn,undefined, « x, y »)).
    2. If v isNaN, return+0𝔽.
    3. Return v.
  5. Let xString be ? ToString(x).
  6. Let yString be ? ToString(y).
  7. Let xSmaller beIsLessThan(xString, yString,true).
  8. If xSmaller istrue, return-1𝔽.
  9. Let ySmaller beIsLessThan(yString, xString,true).
  10. If ySmaller istrue, return1𝔽.
  11. Return+0𝔽.
Note 1

Because non-existent property values always compare greater thanundefinedproperty values, andundefinedalways compares greater than any other value,undefinedproperty values always sort to the end of the result, followed by non-existent property values.

Note 2

Method calls performed by theToStringabstract operationsin steps5and6have the potential to cause SortCompare to not behave as a consistent comparison function.

23.1.3.28 Array.prototype.splice ( start, deleteCount, ...items )

Note 1

The deleteCount elements of the array starting atinteger indexstart are replaced by the elements of items. An Array object containing the deleted elements (if any) is returned.

When the splice method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let relativeStart be ? ToIntegerOrInfinity(start).
  4. If relativeStart is -∞, let actualStart be 0.
  5. Else if relativeStart < 0, let actualStart bemax(len + relativeStart, 0).
  6. Else, let actualStart bemin(relativeStart, len).
  7. If start is not present, then
    1. Let insertCount be 0.
    2. Let actualDeleteCount be 0.
  8. Else if deleteCount is not present, then
    1. Let insertCount be 0.
    2. Let actualDeleteCount be len - actualStart.
  9. Else,
    1. Let insertCount be the number of elements in items.
    2. Let dc be ? ToIntegerOrInfinity(deleteCount).
    3. Let actualDeleteCount be the result ofclampingdc between 0 and len - actualStart.
  10. If len + insertCount - actualDeleteCount > 253 - 1, throw aTypeErrorexception.
  11. Let A be ? ArraySpeciesCreate(O, actualDeleteCount).
  12. Let k be 0.
  13. Repeat, while k < actualDeleteCount,
    1. Let from be ! ToString(𝔽(actualStart + k)).
    2. Let fromPresent be ? HasProperty(O, from).
    3. If fromPresent istrue, then
      1. Let fromValue be ? Get(O, from).
      2. Perform ? CreateDataPropertyOrThrow(A, ! ToString(𝔽(k)), fromValue).
    4. Set k to k + 1.
  14. Perform ? Set(A,"length",𝔽(actualDeleteCount),true).
  15. Let itemCount be the number of elements in items.
  16. If itemCount < actualDeleteCount, then
    1. Set k to actualStart.
    2. Repeat, while k < (len - actualDeleteCount),
      1. Let from be ! ToString(𝔽(k + actualDeleteCount)).
      2. Let to be ! ToString(𝔽(k + itemCount)).
      3. Let fromPresent be ? HasProperty(O, from).
      4. If fromPresent istrue, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue,true).
      5. Else,
        1. Assert: fromPresent isfalse.
        2. Perform ? DeletePropertyOrThrow(O, to).
      6. Set k to k + 1.
    3. Set k to len.
    4. Repeat, while k > (len - actualDeleteCount + itemCount),
      1. Perform ? DeletePropertyOrThrow(O, ! ToString(𝔽(k - 1))).
      2. Set k to k - 1.
  17. Else if itemCount > actualDeleteCount, then
    1. Set k to (len - actualDeleteCount).
    2. Repeat, while k > actualStart,
      1. Let from be ! ToString(𝔽(k + actualDeleteCount - 1)).
      2. Let to be ! ToString(𝔽(k + itemCount - 1)).
      3. Let fromPresent be ? HasProperty(O, from).
      4. If fromPresent istrue, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue,true).
      5. Else,
        1. Assert: fromPresent isfalse.
        2. Perform ? DeletePropertyOrThrow(O, to).
      6. Set k to k - 1.
  18. Set k to actualStart.
  19. For each element E of items, do
    1. Perform ? Set(O, ! ToString(𝔽(k)), E,true).
    2. Set k to k + 1.
  20. Perform ? Set(O,"length",𝔽(len - actualDeleteCount + itemCount),true).
  21. Return A.
Note 2

The explicit setting of the"length"property of the result Array in step20was necessary in previous editions of ECMAScript to ensure that its length was correct in situations where the trailing elements of the result Array were not present. Setting"length"became unnecessary starting in ES2015 when the result Array was initialized to its proper length rather than an empty Array but is carried forward to preserve backward compatibility.

Note 3

The splice function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.29 Array.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement the Array.prototype.toLocaleString method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the ECMA-402 API the following specification of the toLocaleString method is used.

Note 1

The first edition of ECMA-402 did not include a replacement specification for the Array.prototype.toLocaleString method.

The meanings of the optional parameters to this method are defined in the ECMA-402 specification; implementations that do not include ECMA-402 support must not use those parameter positions for anything else.

When the toLocaleString method is called, the following steps are taken:

  1. Let array be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(array).
  3. Let separator be the String value for the list-separator String appropriate for thehost environment's current locale (this is derived in animplementation-definedway).
  4. Let R be the empty String.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. If k > 0, then
      1. Set R to thestring-concatenationof R and separator.
    2. Let nextElement be ? Get(array, ! ToString(𝔽(k))).
    3. If nextElement is notundefinedornull, then
      1. Let S be ? ToString(?Invoke(nextElement,"toLocaleString")).
      2. Set R to thestring-concatenationof R and S.
    4. Set k to k + 1.
  7. Return R.
Note 2

The elements of the array are converted to Strings using their toLocaleString methods, and these Strings are then concatenated, separated by occurrences of a separator String that has been derived in animplementation-definedlocale-specific way. The result of calling this function is intended to be analogous to the result of toString, except that the result of this function is intended to be locale-specific.

Note 3

The toLocaleString function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.30 Array.prototype.toString ( )

When the toString method is called, the following steps are taken:

  1. Let array be ? ToObject(thisvalue).
  2. Let func be ? Get(array,"join").
  3. IfIsCallable(func) isfalse, set func to the intrinsic function %Object.prototype.toString%.
  4. Return ? Call(func, array).
Note

The toString function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.31 Array.prototype.unshift ( ...items )

The arguments are prepended to the start of the array, such that their order within the array is the same as the order in which they appear in the argument list.

When the unshift method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. Let len be ? LengthOfArrayLike(O).
  3. Let argCount be the number of elements in items.
  4. If argCount > 0, then
    1. If len + argCount > 253 - 1, throw aTypeErrorexception.
    2. Let k be len.
    3. Repeat, while k > 0,
      1. Let from be ! ToString(𝔽(k - 1)).
      2. Let to be ! ToString(𝔽(k + argCount - 1)).
      3. Let fromPresent be ? HasProperty(O, from).
      4. If fromPresent istrue, then
        1. Let fromValue be ? Get(O, from).
        2. Perform ? Set(O, to, fromValue,true).
      5. Else,
        1. Assert: fromPresent isfalse.
        2. Perform ? DeletePropertyOrThrow(O, to).
      6. Set k to k - 1.
    4. Let j be+0𝔽.
    5. For each element E of items, do
      1. Perform ? Set(O, ! ToString(j), E,true).
      2. Set j to j +1𝔽.
  5. Perform ? Set(O,"length",𝔽(len + argCount),true).
  6. Return𝔽(len + argCount).

The"length"property of the unshift method is1𝔽.

Note

The unshift function is intentionally generic; it does not require that itsthisvalue be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.

23.1.3.32 Array.prototype.values ( )

When the values method is called, the following steps are taken:

  1. Let O be ? ToObject(thisvalue).
  2. ReturnCreateArrayIterator(O,value).

23.1.3.33 Array.prototype [ @@iterator ] ( )

The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the Array.prototype.values property.

23.1.3.34 Array.prototype [ @@unscopables ]

The initial value of the@@unscopablesdata propertyis an object created by the following steps:

  1. Let unscopableList be ! OrdinaryObjectCreate(null).
  2. Perform ! CreateDataPropertyOrThrow(unscopableList,"copyWithin",true).
  3. Perform ! CreateDataPropertyOrThrow(unscopableList,"entries",true).
  4. Perform ! CreateDataPropertyOrThrow(unscopableList,"fill",true).
  5. Perform ! CreateDataPropertyOrThrow(unscopableList,"find",true).
  6. Perform ! CreateDataPropertyOrThrow(unscopableList,"findIndex",true).
  7. Perform ! CreateDataPropertyOrThrow(unscopableList,"flat",true).
  8. Perform ! CreateDataPropertyOrThrow(unscopableList,"flatMap",true).
  9. Perform ! CreateDataPropertyOrThrow(unscopableList,"includes",true).
  10. Perform ! CreateDataPropertyOrThrow(unscopableList,"keys",true).
  11. Perform ! CreateDataPropertyOrThrow(unscopableList,"values",true).
  12. Return unscopableList.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

Note

The own property names of this object are property names that were not included as standard properties of Array.prototype prior to the ECMAScript 2015 specification. These names are ignored for with statement binding purposes in order to preserve the behaviour of existing code that might use one of these names as a binding in an outer scope that is shadowed by a with statement whose binding object is an Array object.

23.1.4 Properties of Array Instances

Array instances are Array exotic objects and have the internal methods specified for such objects. Array instances inherit properties from theArray prototype object.

Array instances have a"length"property, and a set of enumerable properties witharray indexnames.

23.1.4.1 length

The"length"property of an Array instance is adata propertywhose value is always numerically greater than the name of every configurable own property whose name is anarray index.

The"length"property initially has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.

Note

Reducing the value of the"length"property has the side-effect of deleting own array elements whosearray indexis between the old and new length values. However, non-configurable properties can not be deleted. Attempting to set the"length"property of an Array object to a value that is numerically less than or equal to the largest numeric ownproperty nameof an existing non-configurablearray-indexedproperty of the array will result in the length being set to a numeric value that is one greater than that non-configurable numeric ownproperty name. See10.4.2.1.

23.1.5 Array Iterator Objects

An Array Iterator is an object, that represents a specific iteration over some specific Array instance object. There is not a namedconstructorfor Array Iterator objects. Instead, Array iterator objects are created by calling certain methods of Array instance objects.

23.1.5.1 CreateArrayIterator ( array, kind )

The abstract operation CreateArrayIterator takes arguments array and kind. It is used to create iterator objects for Array methods that return such iterators. It performs the following steps when called:

  1. Assert:Type(array) is Object.
  2. Assert: kind iskey+value,key, orvalue.
  3. Let closure be a newAbstract Closurewith no parameters that captures kind and array and performs the following steps when called:
    1. Let index be 0.
    2. Repeat,
      1. If array has a [[TypedArrayName]] internal slot, then
        1. IfIsDetachedBuffer(array.[[ViewedArrayBuffer]]) istrue, throw aTypeErrorexception.
        2. Let len be array.[[ArrayLength]].
      2. Else,
        1. Let len be ? LengthOfArrayLike(array).
      3. If indexlen, returnundefined.
      4. If kind iskey, perform ? Yield(𝔽(index)).
      5. Else,
        1. Let elementKey be ! ToString(𝔽(index)).
        2. Let elementValue be ? Get(array, elementKey).
        3. If kind isvalue, perform ? Yield(elementValue).
        4. Else,
          1. Assert: kind iskey+value.
          2. Perform ? Yield(!CreateArrayFromList𝔽(index), elementValue »)).
      6. Set index to index + 1.
  4. Return ! CreateIteratorFromClosure(closure,"%ArrayIteratorPrototype%",%ArrayIteratorPrototype%).

23.1.5.2 The %ArrayIteratorPrototype% Object

The %ArrayIteratorPrototype% object:

  • has properties that are inherited by all Array Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has the following properties:

23.1.5.2.1 %ArrayIteratorPrototype%.next ( )

When the next method is called, the following steps are taken:

  1. Return ? GeneratorResume(thisvalue,empty,"%ArrayIteratorPrototype%").

23.1.5.2.2 %ArrayIteratorPrototype% [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Array Iterator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

23.2 TypedArray Objects

TypedArray objects present an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the underlying binary scalar data type that all elements of a TypedArray instance have. There is a distinct TypedArrayconstructor, listed inTable 63, for each of the supported element types. EachconstructorinTable 63has a corresponding distinct prototype object.

Table 63: The TypedArray Constructors
ConstructorName and IntrinsicElement TypeElement SizeConversion OperationDescription
Int8Array
%Int8Array%
Int81ToInt88-bit two's complement signedinteger
Uint8Array
%Uint8Array%
Uint81ToUint88-bit unsignedinteger
Uint8ClampedArray
%Uint8ClampedArray%
Uint8C1ToUint8Clamp8-bit unsignedinteger(clamped conversion)
Int16Array
%Int16Array%
Int162ToInt1616-bit two's complement signedinteger
Uint16Array
%Uint16Array%
Uint162ToUint1616-bit unsignedinteger
Int32Array
%Int32Array%
Int324ToInt3232-bit two's complement signedinteger
Uint32Array
%Uint32Array%
Uint324ToUint3232-bit unsignedinteger
BigInt64Array
%BigInt64Array%
BigInt648ToBigInt6464-bit two's complement signedinteger
BigUint64Array
%BigUint64Array%
BigUint648ToBigUint6464-bit unsignedinteger
Float32Array
%Float32Array%
Float32432-bit IEEE floating point
Float64Array
%Float64Array%
Float64864-bit IEEE floating point

In the definitions below, references to TypedArray should be replaced with the appropriateconstructorname from the above table.

23.2.1 The %TypedArray% Intrinsic Object

The %TypedArray% intrinsic object:

  • is aconstructorfunction objectthat all of the TypedArrayconstructorobjects inherit from.
  • along with its corresponding prototype object, provides common properties that are inherited by all TypedArray constructors and their instances.
  • does not have a global name or appear as a property of theglobal object.
  • acts as the abstract superclass of the various TypedArray constructors.
  • will throw an error when invoked, because it is an abstract classconstructor. The TypedArray constructors do not perform a super call to it.

23.2.1.1 %TypedArray% ( )

The%TypedArray%constructorperforms the following steps when called:

  1. Throw aTypeErrorexception.

The"length"property of the%TypedArray%constructorfunction is+0𝔽.

23.2.2 Properties of the %TypedArray% Intrinsic Object

The%TypedArray%intrinsic object:

  • has a [[Prototype]] internal slot whose value is%Function.prototype%.
  • has a"name"property whose value is"TypedArray".
  • has the following properties:

23.2.2.1 %TypedArray%.from ( source [ , mapfn [ , thisArg ] ] )

When the from method is called, the following steps are taken:

  1. Let C be thethisvalue.
  2. IfIsConstructor(C) isfalse, throw aTypeErrorexception.
  3. If mapfn isundefined, let mapping befalse.
  4. Else,
    1. IfIsCallable(mapfn) isfalse, throw aTypeErrorexception.
    2. Let mapping betrue.
  5. Let usingIterator be ? GetMethod(source,@@iterator).
  6. If usingIterator is notundefined, then
    1. Let values be ? IterableToList(source, usingIterator).
    2. Let len be the number of elements in values.
    3. Let targetObj be ? TypedArrayCreate(C, «𝔽(len) »).
    4. Let k be 0.
    5. Repeat, while k < len,
      1. Let Pk be ! ToString(𝔽(k)).
      2. Let kValue be the first element of values and remove that element from values.
      3. If mapping istrue, then
        1. Let mappedValue be ? Call(mapfn, thisArg, « kValue,𝔽(k) »).
      4. Else, let mappedValue be kValue.
      5. Perform ? Set(targetObj, Pk, mappedValue,true).
      6. Set k to k + 1.
    6. Assert: values is now an emptyList.
    7. Return targetObj.
  7. NOTE: source is not an Iterable so assume it is already anarray-like object.
  8. Let arrayLike be ! ToObject(source).
  9. Let len be ? LengthOfArrayLike(arrayLike).
  10. Let targetObj be ? TypedArrayCreate(C, «𝔽(len) »).
  11. Let k be 0.
  12. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. If mapping istrue, then
      1. Let mappedValue be ? Call(mapfn, thisArg, « kValue,𝔽(k) »).
    4. Else, let mappedValue be kValue.
    5. Perform ? Set(targetObj, Pk, mappedValue,true).
    6. Set k to k + 1.
  13. Return targetObj.

23.2.2.2 %TypedArray%.of ( ...items )

When the of method is called, the following steps are taken:

  1. Let len be the number of elements in items.
  2. Let C be thethisvalue.
  3. IfIsConstructor(C) isfalse, throw aTypeErrorexception.
  4. Let newObj be ? TypedArrayCreate(C, «𝔽(len) »).
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let kValue be items[k].
    2. Let Pk be ! ToString(𝔽(k)).
    3. Perform ? Set(newObj, Pk, kValue,true).
    4. Set k to k + 1.
  7. Return newObj.

23.2.2.3 %TypedArray%.prototype

The initial value of%TypedArray%.prototype is the%TypedArray% prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

23.2.2.4 get %TypedArray% [ @@species ]

%TypedArray%[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

%TypedArray.prototype%methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.

23.2.3 Properties of the %TypedArray% Prototype Object

The %TypedArray% prototype object:

  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is %TypedArray.prototype%.
  • is anordinary object.
  • does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

23.2.3.1 get %TypedArray%.prototype.buffer

%TypedArray%.prototype.buffer is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Return buffer.

23.2.3.2 get %TypedArray%.prototype.byteLength

%TypedArray%.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. IfIsDetachedBuffer(buffer) istrue, return+0𝔽.
  6. Let size be O.[[ByteLength]].
  7. Return𝔽(size).

23.2.3.3 get %TypedArray%.prototype.byteOffset

%TypedArray%.prototype.byteOffset is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. IfIsDetachedBuffer(buffer) istrue, return+0𝔽.
  6. Let offset be O.[[ByteOffset]].
  7. Return𝔽(offset).

23.2.3.4 %TypedArray%.prototype.constructor

The initial value of%TypedArray%.prototype.constructor is the%TypedArray%intrinsic object.

23.2.3.5 %TypedArray%.prototype.copyWithin ( target, start [ , end ] )

The interpretation and use of the arguments of%TypedArray%.prototype.copyWithin are the same as for Array.prototype.copyWithin as defined in23.1.3.3.

When the copyWithin method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeTarget be ? ToIntegerOrInfinity(target).
  5. If relativeTarget is -∞, let to be 0.
  6. Else if relativeTarget < 0, let to bemax(len + relativeTarget, 0).
  7. Else, let to bemin(relativeTarget, len).
  8. Let relativeStart be ? ToIntegerOrInfinity(start).
  9. If relativeStart is -∞, let from be 0.
  10. Else if relativeStart < 0, let from bemax(len + relativeStart, 0).
  11. Else, let from bemin(relativeStart, len).
  12. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  13. If relativeEnd is -∞, let final be 0.
  14. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  15. Else, let final bemin(relativeEnd, len).
  16. Let count bemin(final - from, len - to).
  17. If count > 0, then
    1. NOTE: The copying must be performed in a manner that preserves the bit-level encoding of the source data.
    2. Let buffer be O.[[ViewedArrayBuffer]].
    3. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
    4. Let typedArrayName be the String value of O.[[TypedArrayName]].
    5. Let elementSize be the Element Size value specified inTable 63for typedArrayName.
    6. Let byteOffset be O.[[ByteOffset]].
    7. Let toByteIndex be to × elementSize + byteOffset.
    8. Let fromByteIndex be from × elementSize + byteOffset.
    9. Let countBytes be count × elementSize.
    10. If fromByteIndex < toByteIndex and toByteIndex < fromByteIndex + countBytes, then
      1. Let direction be -1.
      2. Set fromByteIndex to fromByteIndex + countBytes - 1.
      3. Set toByteIndex to toByteIndex + countBytes - 1.
    11. Else,
      1. Let direction be 1.
    12. Repeat, while countBytes > 0,
      1. Let value beGetValueFromBuffer(buffer, fromByteIndex,Uint8,true,Unordered).
      2. PerformSetValueInBuffer(buffer, toByteIndex,Uint8, value,true,Unordered).
      3. Set fromByteIndex to fromByteIndex + direction.
      4. Set toByteIndex to toByteIndex + direction.
      5. Set countBytes to countBytes - 1.
  18. Return O.

23.2.3.6 %TypedArray%.prototype.entries ( )

When the entries method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. ReturnCreateArrayIterator(O,key+value).

23.2.3.7 %TypedArray%.prototype.every ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.every are the same as for Array.prototype.every as defined in23.1.3.5.

When the every method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult isfalse, returnfalse.
    5. Set k to k + 1.
  7. Returntrue.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.8 %TypedArray%.prototype.fill ( value [ , start [ , end ] ] )

The interpretation and use of the arguments of%TypedArray%.prototype.fill are the same as for Array.prototype.fill as defined in23.1.3.6.

When the fill method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If O.[[ContentType]] isBigInt, set value to ? ToBigInt(value).
  5. Otherwise, set value to ? ToNumber(value).
  6. Let relativeStart be ? ToIntegerOrInfinity(start).
  7. If relativeStart is -∞, let k be 0.
  8. Else if relativeStart < 0, let k bemax(len + relativeStart, 0).
  9. Else, let k bemin(relativeStart, len).
  10. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd is -∞, let final be 0.
  12. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  13. Else, let final bemin(relativeEnd, len).
  14. IfIsDetachedBuffer(O.[[ViewedArrayBuffer]]) istrue, throw aTypeErrorexception.
  15. Repeat, while k < final,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Perform ! Set(O, Pk, value,true).
    3. Set k to k + 1.
  16. Return O.

23.2.3.9 %TypedArray%.prototype.filter ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.filter are the same as for Array.prototype.filter as defined in23.1.3.7.

When the filter method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. Let kept be a new emptyList.
  6. Let k be 0.
  7. Let captured be 0.
  8. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let selected be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
    4. If selected istrue, then
      1. Append kValue to the end of kept.
      2. Set captured to captured + 1.
    5. Set k to k + 1.
  9. Let A be ? TypedArraySpeciesCreate(O, «𝔽(captured) »).
  10. Let n be 0.
  11. For each element e of kept, do
    1. Perform ! Set(A, ! ToString(𝔽(n)), e,true).
    2. Set n to n + 1.
  12. Return A.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.10 %TypedArray%.prototype.find ( predicate [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.find are the same as for Array.prototype.find as defined in23.1.3.8.

When the find method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(predicate) isfalse, throw aTypeErrorexception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(predicate, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult istrue, return kValue.
    5. Set k to k + 1.
  7. Returnundefined.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.11 %TypedArray%.prototype.findIndex ( predicate [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.findIndex are the same as for Array.prototype.findIndex as defined in23.1.3.9.

When the findIndex method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(predicate) isfalse, throw aTypeErrorexception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(predicate, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult istrue, return𝔽(k).
    5. Set k to k + 1.
  7. Return-1𝔽.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.12 %TypedArray%.prototype.forEach ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.forEach are the same as for Array.prototype.forEach as defined in23.1.3.12.

When the forEach method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Perform ? Call(callbackfn, thisArg, « kValue,𝔽(k), O »).
    4. Set k to k + 1.
  7. Returnundefined.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.13 %TypedArray%.prototype.includes ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of%TypedArray%.prototype.includes are the same as for Array.prototype.includes as defined in23.1.3.13.

When the includes method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len is 0, returnfalse.
  5. Let n be ? ToIntegerOrInfinity(fromIndex).
  6. Assert: If fromIndex isundefined, then n is 0.
  7. If n is +∞, returnfalse.
  8. Else if n is -∞, set n to 0.
  9. If n ≥ 0, then
    1. Let k be n.
  10. Else,
    1. Let k be len + n.
    2. If k < 0, set k to 0.
  11. Repeat, while k < len,
    1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
    2. IfSameValueZero(searchElement, elementK) istrue, returntrue.
    3. Set k to k + 1.
  12. Returnfalse.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.14 %TypedArray%.prototype.indexOf ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of%TypedArray%.prototype.indexOf are the same as for Array.prototype.indexOf as defined in23.1.3.14.

When the indexOf method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len is 0, return-1𝔽.
  5. Let n be ? ToIntegerOrInfinity(fromIndex).
  6. Assert: If fromIndex isundefined, then n is 0.
  7. If n is +∞, return-1𝔽.
  8. Else if n is -∞, set n to 0.
  9. If n ≥ 0, then
    1. Let k be n.
  10. Else,
    1. Let k be len + n.
    2. If k < 0, set k to 0.
  11. Repeat, while k < len,
    1. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent istrue, then
      1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
      2. Let same beIsStrictlyEqual(searchElement, elementK).
      3. If same istrue, return𝔽(k).
    3. Set k to k + 1.
  12. Return-1𝔽.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.15 %TypedArray%.prototype.join ( separator )

The interpretation and use of the arguments of%TypedArray%.prototype.join are the same as for Array.prototype.join as defined in23.1.3.15.

When the join method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If separator isundefined, let sep be the single-element String",".
  5. Else, let sep be ? ToString(separator).
  6. Let R be the empty String.
  7. Let k be 0.
  8. Repeat, while k < len,
    1. If k > 0, set R to thestring-concatenationof R and sep.
    2. Let element be ! Get(O, ! ToString(𝔽(k))).
    3. If element isundefined, let next be the empty String; otherwise, let next be ! ToString(element).
    4. Set R to thestring-concatenationof R and next.
    5. Set k to k + 1.
  9. Return R.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.16 %TypedArray%.prototype.keys ( )

When the keys method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. ReturnCreateArrayIterator(O,key).

23.2.3.17 %TypedArray%.prototype.lastIndexOf ( searchElement [ , fromIndex ] )

The interpretation and use of the arguments of%TypedArray%.prototype.lastIndexOf are the same as for Array.prototype.lastIndexOf as defined in23.1.3.17.

When the lastIndexOf method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. If len is 0, return-1𝔽.
  5. If fromIndex is present, let n be ? ToIntegerOrInfinity(fromIndex); else let n be len - 1.
  6. If n is -∞, return-1𝔽.
  7. If n ≥ 0, then
    1. Let k bemin(n, len - 1).
  8. Else,
    1. Let k be len + n.
  9. Repeat, while k ≥ 0,
    1. Let kPresent be ! HasProperty(O, ! ToString(𝔽(k))).
    2. If kPresent istrue, then
      1. Let elementK be ! Get(O, ! ToString(𝔽(k))).
      2. Let same beIsStrictlyEqual(searchElement, elementK).
      3. If same istrue, return𝔽(k).
    3. Set k to k - 1.
  10. Return-1𝔽.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.18 get %TypedArray%.prototype.length

%TypedArray%.prototype.length is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has [[ViewedArrayBuffer]] and [[ArrayLength]] internal slots.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. IfIsDetachedBuffer(buffer) istrue, return+0𝔽.
  6. Let length be O.[[ArrayLength]].
  7. Return𝔽(length).

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.19 %TypedArray%.prototype.map ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.map are the same as for Array.prototype.map as defined in23.1.3.18.

When the map method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. Let A be ? TypedArraySpeciesCreate(O, «𝔽(len) »).
  6. Let k be 0.
  7. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let mappedValue be ? Call(callbackfn, thisArg, « kValue,𝔽(k), O »).
    4. Perform ? Set(A, Pk, mappedValue,true).
    5. Set k to k + 1.
  8. Return A.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.20 %TypedArray%.prototype.reduce ( callbackfn [ , initialValue ] )

The interpretation and use of the arguments of%TypedArray%.prototype.reduce are the same as for Array.prototype.reduce as defined in23.1.3.21.

When the reduce method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. If len = 0 and initialValue is not present, throw aTypeErrorexception.
  6. Let k be 0.
  7. Let accumulator beundefined.
  8. If initialValue is present, then
    1. Set accumulator to initialValue.
  9. Else,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Set accumulator to ! Get(O, Pk).
    3. Set k to k + 1.
  10. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
    4. Set k to k + 1.
  11. Return accumulator.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.21 %TypedArray%.prototype.reduceRight ( callbackfn [ , initialValue ] )

The interpretation and use of the arguments of%TypedArray%.prototype.reduceRight are the same as for Array.prototype.reduceRight as defined in23.1.3.22.

When the reduceRight method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. If len is 0 and initialValue is not present, throw aTypeErrorexception.
  6. Let k be len - 1.
  7. Let accumulator beundefined.
  8. If initialValue is present, then
    1. Set accumulator to initialValue.
  9. Else,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Set accumulator to ! Get(O, Pk).
    3. Set k to k - 1.
  10. Repeat, while k ≥ 0,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Set accumulator to ? Call(callbackfn,undefined, « accumulator, kValue,𝔽(k), O »).
    4. Set k to k - 1.
  11. Return accumulator.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.22 %TypedArray%.prototype.reverse ( )

The interpretation and use of the arguments of%TypedArray%.prototype.reverse are the same as for Array.prototype.reverse as defined in23.1.3.23.

When the reverse method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let middle befloor(len / 2).
  5. Let lower be 0.
  6. Repeat, while lowermiddle,
    1. Let upper be len - lower - 1.
    2. Let upperP be ! ToString(𝔽(upper)).
    3. Let lowerP be ! ToString(𝔽(lower)).
    4. Let lowerValue be ! Get(O, lowerP).
    5. Let upperValue be ! Get(O, upperP).
    6. Perform ! Set(O, lowerP, upperValue,true).
    7. Perform ! Set(O, upperP, lowerValue,true).
    8. Set lower to lower + 1.
  7. Return O.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.23 %TypedArray%.prototype.set ( source [ , offset ] )

%TypedArray%.prototype.set is a function whose behaviour differs based upon the type of its first argument.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

Sets multiple values in this TypedArray, reading the values from source. The optional offset value indicates the first element index in this TypedArray where values are written. If omitted, it is assumed to be 0.

When the set method is called, the following steps are taken:

  1. Let target be thethisvalue.
  2. Perform ? RequireInternalSlot(target, [[TypedArrayName]]).
  3. Assert: target has a [[ViewedArrayBuffer]] internal slot.
  4. Let targetOffset be ? ToIntegerOrInfinity(offset).
  5. If targetOffset < 0, throw aRangeErrorexception.
  6. If source is an Object that has a [[TypedArrayName]] internal slot, then
    1. Perform ? SetTypedArrayFromTypedArray(target, targetOffset, source).
  7. Else,
    1. Perform ? SetTypedArrayFromArrayLike(target, targetOffset, source).
  8. Returnundefined.

23.2.3.23.1 SetTypedArrayFromTypedArray ( target, targetOffset, source )

The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray object), targetOffset (a non-negativeintegeror +∞), and source (a TypedArray object). It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:

  1. Assert: source is an Object that has a [[TypedArrayName]] internal slot.
  2. Let targetBuffer be target.[[ViewedArrayBuffer]].
  3. IfIsDetachedBuffer(targetBuffer) istrue, throw aTypeErrorexception.
  4. Let targetLength be target.[[ArrayLength]].
  5. Let srcBuffer be source.[[ViewedArrayBuffer]].
  6. IfIsDetachedBuffer(srcBuffer) istrue, throw aTypeErrorexception.
  7. Let targetName be the String value of target.[[TypedArrayName]].
  8. Let targetType be the Element Type value inTable 63for targetName.
  9. Let targetElementSize be the Element Size value specified inTable 63for targetName.
  10. Let targetByteOffset be target.[[ByteOffset]].
  11. Let srcName be the String value of source.[[TypedArrayName]].
  12. Let srcType be the Element Type value inTable 63for srcName.
  13. Let srcElementSize be the Element Size value specified inTable 63for srcName.
  14. Let srcLength be source.[[ArrayLength]].
  15. Let srcByteOffset be source.[[ByteOffset]].
  16. If targetOffset is +∞, throw aRangeErrorexception.
  17. If srcLength + targetOffset > targetLength, throw aRangeErrorexception.
  18. If target.[[ContentType]] ≠ source.[[ContentType]], throw aTypeErrorexception.
  19. If bothIsSharedArrayBuffer(srcBuffer) andIsSharedArrayBuffer(targetBuffer) aretrue, then
    1. If srcBuffer.[[ArrayBufferData]] and targetBuffer.[[ArrayBufferData]] are the sameShared Data Blockvalues, let same betrue; else let same befalse.
  20. Else, let same beSameValue(srcBuffer, targetBuffer).
  21. If same istrue, then
    1. Let srcByteLength be source.[[ByteLength]].
    2. Set srcBuffer to ? CloneArrayBuffer(srcBuffer, srcByteOffset, srcByteLength,%ArrayBuffer%).
    3. NOTE:%ArrayBuffer%is used to clone srcBuffer because is it known to not have any observable side-effects.
    4. Let srcByteIndex be 0.
  22. Else, let srcByteIndex be srcByteOffset.
  23. Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
  24. Let limit be targetByteIndex + targetElementSize × srcLength.
  25. If srcType is the same as targetType, then
    1. NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
    2. Repeat, while targetByteIndex < limit,
      1. Let value beGetValueFromBuffer(srcBuffer, srcByteIndex,Uint8,true,Unordered).
      2. PerformSetValueInBuffer(targetBuffer, targetByteIndex,Uint8, value,true,Unordered).
      3. Set srcByteIndex to srcByteIndex + 1.
      4. Set targetByteIndex to targetByteIndex + 1.
  26. Else,
    1. Repeat, while targetByteIndex < limit,
      1. Let value beGetValueFromBuffer(srcBuffer, srcByteIndex, srcType,true,Unordered).
      2. PerformSetValueInBuffer(targetBuffer, targetByteIndex, targetType, value,true,Unordered).
      3. Set srcByteIndex to srcByteIndex + srcElementSize.
      4. Set targetByteIndex to targetByteIndex + targetElementSize.

23.2.3.23.2 SetTypedArrayFromArrayLike ( target, targetOffset, source )

The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray object), targetOffset (a non-negativeintegeror +∞), and source (an ECMAScript value other than a TypedArray object). It sets multiple values in target, starting at index targetOffset, reading the values from source. It performs the following steps when called:

  1. Assert: source is anyECMAScript language valueother than an Object with a [[TypedArrayName]] internal slot.
  2. Let targetBuffer be target.[[ViewedArrayBuffer]].
  3. IfIsDetachedBuffer(targetBuffer) istrue, throw aTypeErrorexception.
  4. Let targetLength be target.[[ArrayLength]].
  5. Let targetName be the String value of target.[[TypedArrayName]].
  6. Let targetElementSize be the Element Size value specified inTable 63for targetName.
  7. Let targetType be the Element Type value inTable 63for targetName.
  8. Let targetByteOffset be target.[[ByteOffset]].
  9. Let src be ? ToObject(source).
  10. Let srcLength be ? LengthOfArrayLike(src).
  11. If targetOffset is +∞, throw aRangeErrorexception.
  12. If srcLength + targetOffset > targetLength, throw aRangeErrorexception.
  13. Let targetByteIndex be targetOffset × targetElementSize + targetByteOffset.
  14. Let k be 0.
  15. Let limit be targetByteIndex + targetElementSize × srcLength.
  16. Repeat, while targetByteIndex < limit,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let value be ? Get(src, Pk).
    3. If target.[[ContentType]] isBigInt, set value to ? ToBigInt(value).
    4. Otherwise, set value to ? ToNumber(value).
    5. IfIsDetachedBuffer(targetBuffer) istrue, throw aTypeErrorexception.
    6. PerformSetValueInBuffer(targetBuffer, targetByteIndex, targetType, value,true,Unordered).
    7. Set k to k + 1.
    8. Set targetByteIndex to targetByteIndex + targetElementSize.

23.2.3.24 %TypedArray%.prototype.slice ( start, end )

The interpretation and use of the arguments of%TypedArray%.prototype.slice are the same as for Array.prototype.slice as defined in23.1.3.25. The following steps are taken:

When the slice method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. Let relativeStart be ? ToIntegerOrInfinity(start).
  5. If relativeStart is -∞, let k be 0.
  6. Else if relativeStart < 0, let k bemax(len + relativeStart, 0).
  7. Else, let k bemin(relativeStart, len).
  8. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  9. If relativeEnd is -∞, let final be 0.
  10. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  11. Else, let final bemin(relativeEnd, len).
  12. Let count bemax(final - k, 0).
  13. Let A be ? TypedArraySpeciesCreate(O, «𝔽(count) »).
  14. If count > 0, then
    1. IfIsDetachedBuffer(O.[[ViewedArrayBuffer]]) istrue, throw aTypeErrorexception.
    2. Let srcName be the String value of O.[[TypedArrayName]].
    3. Let srcType be the Element Type value inTable 63for srcName.
    4. Let targetName be the String value of A.[[TypedArrayName]].
    5. Let targetType be the Element Type value inTable 63for targetName.
    6. If srcType is different from targetType, then
      1. Let n be 0.
      2. Repeat, while k < final,
        1. Let Pk be ! ToString(𝔽(k)).
        2. Let kValue be ! Get(O, Pk).
        3. Perform ! Set(A, ! ToString(𝔽(n)), kValue,true).
        4. Set k to k + 1.
        5. Set n to n + 1.
    7. Else,
      1. Let srcBuffer be O.[[ViewedArrayBuffer]].
      2. Let targetBuffer be A.[[ViewedArrayBuffer]].
      3. Let elementSize be the Element Size value specified inTable 63for Element Type srcType.
      4. NOTE: If srcType and targetType are the same, the transfer must be performed in a manner that preserves the bit-level encoding of the source data.
      5. Let srcByteOffset be O.[[ByteOffset]].
      6. Let targetByteIndex be A.[[ByteOffset]].
      7. Let srcByteIndex be (k × elementSize) + srcByteOffset.
      8. Let limit be targetByteIndex + count × elementSize.
      9. Repeat, while targetByteIndex < limit,
        1. Let value beGetValueFromBuffer(srcBuffer, srcByteIndex,Uint8,true,Unordered).
        2. PerformSetValueInBuffer(targetBuffer, targetByteIndex,Uint8, value,true,Unordered).
        3. Set srcByteIndex to srcByteIndex + 1.
        4. Set targetByteIndex to targetByteIndex + 1.
  15. Return A.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.25 %TypedArray%.prototype.some ( callbackfn [ , thisArg ] )

The interpretation and use of the arguments of%TypedArray%.prototype.some are the same as for Array.prototype.some as defined in23.1.3.26.

When the some method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. Let len be O.[[ArrayLength]].
  4. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  5. Let k be 0.
  6. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ! Get(O, Pk).
    3. Let testResult be ! ToBoolean(?Call(callbackfn, thisArg, « kValue,𝔽(k), O »)).
    4. If testResult istrue, returntrue.
    5. Set k to k + 1.
  7. Returnfalse.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.26 %TypedArray%.prototype.sort ( comparefn )

%TypedArray%.prototype.sort is a distinct function that, except as described below, implements the same requirements as those of Array.prototype.sort as defined in23.1.3.27. The implementation of the%TypedArray%.prototype.sort specification may be optimized with the knowledge that thethisvalue is an object that has a fixed length and whoseinteger-indexedproperties are not sparse.

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

Upon entry, the following steps are performed to initialize evaluation of the sort function. These steps are used instead of steps13in23.1.3.27:

  1. If comparefn is notundefinedandIsCallable(comparefn) isfalse, throw aTypeErrorexception.
  2. Let obj be thethisvalue.
  3. Perform ? ValidateTypedArray(obj).
  4. Let buffer be obj.[[ViewedArrayBuffer]].
  5. Let len be obj.[[ArrayLength]].

The following version ofSortCompareis used by%TypedArray%.prototype.sort. It performs a numeric comparison rather than the string comparison used in23.1.3.27.

The abstract operation TypedArraySortCompare takes arguments x and y. It also has access to the comparefn and buffer values of the current invocation of the sort method. It performs the following steps when called:

  1. Assert: BothType(x) andType(y) are Number or both are BigInt.
  2. If comparefn is notundefined, then
    1. Let v be ? ToNumber(?Call(comparefn,undefined, « x, y »)).
    2. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
    3. If v isNaN, return+0𝔽.
    4. Return v.
  3. If x and y are bothNaN, return+0𝔽.
  4. If x isNaN, return1𝔽.
  5. If y isNaN, return-1𝔽.
  6. If x < y, return-1𝔽.
  7. If x > y, return1𝔽.
  8. If x is-0𝔽 and y is+0𝔽, return-1𝔽.
  9. If x is+0𝔽 and y is-0𝔽, return1𝔽.
  10. Return+0𝔽.
Note

BecauseNaNalways compares greater than any other value,NaNproperty values always sort to the end of the result when comparefn is not provided.

23.2.3.27 %TypedArray%.prototype.subarray ( begin, end )

Returns a new TypedArray object whose element type is the same as this TypedArray and whose ArrayBuffer is the same as the ArrayBuffer of this TypedArray, referencing the elements at begin, inclusive, up to end, exclusive. If either begin or end is negative, it refers to an index from the end of the array, as opposed to from the beginning.

When the subarray method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Let srcLength be O.[[ArrayLength]].
  6. Let relativeBegin be ? ToIntegerOrInfinity(begin).
  7. If relativeBegin is -∞, let beginIndex be 0.
  8. Else if relativeBegin < 0, let beginIndex bemax(srcLength + relativeBegin, 0).
  9. Else, let beginIndex bemin(relativeBegin, srcLength).
  10. If end isundefined, let relativeEnd be srcLength; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd is -∞, let endIndex be 0.
  12. Else if relativeEnd < 0, let endIndex bemax(srcLength + relativeEnd, 0).
  13. Else, let endIndex bemin(relativeEnd, srcLength).
  14. Let newLength bemax(endIndex - beginIndex, 0).
  15. Let constructorName be the String value of O.[[TypedArrayName]].
  16. Let elementSize be the Element Size value specified inTable 63for constructorName.
  17. Let srcByteOffset be O.[[ByteOffset]].
  18. Let beginByteOffset be srcByteOffset + beginIndex × elementSize.
  19. Let argumentsList be « buffer,𝔽(beginByteOffset),𝔽(newLength) ».
  20. Return ? TypedArraySpeciesCreate(O, argumentsList).

This function is not generic. Thethisvalue must be an object with a [[TypedArrayName]] internal slot.

23.2.3.28 %TypedArray%.prototype.toLocaleString ( [ reserved1 [ , reserved2 ] ] )

%TypedArray%.prototype.toLocaleString is a distinct function that implements the same algorithm as Array.prototype.toLocaleString as defined in23.1.3.29except that thethisvalue's [[ArrayLength]] internal slot is accessed in place of performing a [[Get]] of"length". The implementation of the algorithm may be optimized with the knowledge that thethisvalue is an object that has a fixed length and whoseinteger-indexedproperties are not sparse. However, such optimization must not introduce any observable changes in the specified behaviour of the algorithm.

This function is not generic.ValidateTypedArrayis applied to thethisvalue prior to evaluating the algorithm. If its result is anabrupt completionthat exception is thrown instead of evaluating the algorithm.

Note

If the ECMAScript implementation includes the ECMA-402 Internationalization API this function is based upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402 specification.

23.2.3.29 %TypedArray%.prototype.toString ( )

The initial value of the%TypedArray%.prototype.toStringdata propertyis the same built-infunction objectas the Array.prototype.toString method defined in23.1.3.30.

23.2.3.30 %TypedArray%.prototype.values ( )

When the values method is called, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? ValidateTypedArray(O).
  3. ReturnCreateArrayIterator(O,value).

23.2.3.31 %TypedArray%.prototype [ @@iterator ] ( )

The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the%TypedArray%.prototype.values property.

23.2.3.32 get %TypedArray%.prototype [ @@toStringTag ]

%TypedArray%.prototype[@@toStringTag] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps when called:

  1. Let O be thethisvalue.
  2. IfType(O) is not Object, returnundefined.
  3. If O does not have a [[TypedArrayName]] internal slot, returnundefined.
  4. Let name be O.[[TypedArrayName]].
  5. Assert:Type(name) is String.
  6. Return name.

This property has the attributes { [[Enumerable]]:false, [[Configurable]]:true}.

The initial value of the"name"property of this function is"get [Symbol.toStringTag]".

23.2.4 Abstract Operations for TypedArray Objects

23.2.4.1 TypedArraySpeciesCreate ( exemplar, argumentList )

The abstract operation TypedArraySpeciesCreate takes arguments exemplar and argumentList. It is used to specify the creation of a new TypedArray object using aconstructorfunction that is derived from exemplar. It performs the following steps when called:

  1. Assert: exemplar is an Object that has [[TypedArrayName]] and [[ContentType]] internal slots.
  2. Let defaultConstructor be the intrinsic object listed in column one ofTable 63for exemplar.[[TypedArrayName]].
  3. Let constructor be ? SpeciesConstructor(exemplar, defaultConstructor).
  4. Let result be ? TypedArrayCreate(constructor, argumentList).
  5. Assert: result has [[TypedArrayName]] and [[ContentType]] internal slots.
  6. If result.[[ContentType]] ≠ exemplar.[[ContentType]], throw aTypeErrorexception.
  7. Return result.

23.2.4.2 TypedArrayCreate ( constructor, argumentList )

The abstract operation TypedArrayCreate takes arguments constructor and argumentList. It is used to specify the creation of a new TypedArray object using aconstructorfunction. It performs the following steps when called:

  1. Let newTypedArray be ? Construct(constructor, argumentList).
  2. Perform ? ValidateTypedArray(newTypedArray).
  3. If argumentList is aListof a single Number, then
    1. If newTypedArray.[[ArrayLength]] <(argumentList[0]), throw aTypeErrorexception.
  4. Return newTypedArray.

23.2.4.3 ValidateTypedArray ( O )

The abstract operation ValidateTypedArray takes argument O. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(O, [[TypedArrayName]]).
  2. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  3. Let buffer be O.[[ViewedArrayBuffer]].
  4. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.

23.2.5 The TypedArray Constructors

Each TypedArrayconstructor:

  • is an intrinsic object that has the structure described below, differing only in the name used as theconstructorname instead of TypedArray, inTable 63.
  • is a function whose behaviour differs based upon the number and types of its arguments. The actual behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed to it.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified TypedArray behaviour must include a super call to the TypedArrayconstructorto create and initialize the subclass instance with the internal state necessary to support the%TypedArray%.prototype built-in methods.
  • has a"length"property whose value is3𝔽.

23.2.5.1 TypedArray ( ...args )

Each TypedArrayconstructorperforms the following steps when called:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let constructorName be the String value of theConstructorName value specified inTable 63for this TypedArrayconstructor.
  3. Let proto be "%TypedArray.prototype%".
  4. Let numberOfArgs be the number of elements in args.
  5. If numberOfArgs = 0, then
    1. Return ? AllocateTypedArray(constructorName, NewTarget, proto, 0).
  6. Else,
    1. Let firstArgument be args[0].
    2. IfType(firstArgument) is Object, then
      1. Let O be ? AllocateTypedArray(constructorName, NewTarget, proto).
      2. If firstArgument has a [[TypedArrayName]] internal slot, then
        1. Perform ? InitializeTypedArrayFromTypedArray(O, firstArgument).
      3. Else if firstArgument has an [[ArrayBufferData]] internal slot, then
        1. If numberOfArgs > 1, let byteOffset be args[1]; else let byteOffset beundefined.
        2. If numberOfArgs > 2, let length be args[2]; else let length beundefined.
        3. Perform ? InitializeTypedArrayFromArrayBuffer(O, firstArgument, byteOffset, length).
      4. Else,
        1. Assert:Type(firstArgument) is Object and firstArgument does not have either a [[TypedArrayName]] or an [[ArrayBufferData]] internal slot.
        2. Let usingIterator be ? GetMethod(firstArgument,@@iterator).
        3. If usingIterator is notundefined, then
          1. Let values be ? IterableToList(firstArgument, usingIterator).
          2. Perform ? InitializeTypedArrayFromList(O, values).
        4. Else,
          1. NOTE: firstArgument is not an Iterable so assume it is already anarray-like object.
          2. Perform ? InitializeTypedArrayFromArrayLike(O, firstArgument).
      5. Return O.
    3. Else,
      1. Assert: firstArgument is not an Object.
      2. Let elementLength be ? ToIndex(firstArgument).
      3. Return ? AllocateTypedArray(constructorName, NewTarget, proto, elementLength).

23.2.5.1.1 AllocateTypedArray ( constructorName, newTarget, defaultProto [ , length ] )

The abstract operation AllocateTypedArray takes arguments constructorName (a String which is the name of a TypedArrayconstructorinTable 63), newTarget, and defaultProto and optional argument length (a non-negativeinteger). It is used to validate and create an instance of a TypedArrayconstructor. If the length argument is passed, an ArrayBuffer of that length is also allocated and associated with the new TypedArray instance. AllocateTypedArray provides common semantics that is used by TypedArray. It performs the following steps when called:

  1. Let proto be ? GetPrototypeFromConstructor(newTarget, defaultProto).
  2. Let obj be ! IntegerIndexedObjectCreate(proto).
  3. Assert: obj.[[ViewedArrayBuffer]] isundefined.
  4. Set obj.[[TypedArrayName]] to constructorName.
  5. If constructorName is"BigInt64Array"or"BigUint64Array", set obj.[[ContentType]] toBigInt.
  6. Otherwise, set obj.[[ContentType]] toNumber.
  7. If length is not present, then
    1. Set obj.[[ByteLength]] to 0.
    2. Set obj.[[ByteOffset]] to 0.
    3. Set obj.[[ArrayLength]] to 0.
  8. Else,
    1. Perform ? AllocateTypedArrayBuffer(obj, length).
  9. Return obj.

23.2.5.1.2 InitializeTypedArrayFromTypedArray ( O, srcArray )

The abstract operation InitializeTypedArrayFromTypedArray takes arguments O (a TypedArray object) and srcArray (a TypedArray object). It performs the following steps when called:

  1. Assert: O is an Object that has a [[TypedArrayName]] internal slot.
  2. Assert: srcArray is an Object that has a [[TypedArrayName]] internal slot.
  3. Let srcData be srcArray.[[ViewedArrayBuffer]].
  4. IfIsDetachedBuffer(srcData) istrue, throw aTypeErrorexception.
  5. Let constructorName be the String value of O.[[TypedArrayName]].
  6. Let elementType be the Element Type value inTable 63for constructorName.
  7. Let elementLength be srcArray.[[ArrayLength]].
  8. Let srcName be the String value of srcArray.[[TypedArrayName]].
  9. Let srcType be the Element Type value inTable 63for srcName.
  10. Let srcElementSize be the Element Size value specified inTable 63for srcName.
  11. Let srcByteOffset be srcArray.[[ByteOffset]].
  12. Let elementSize be the Element Size value specified inTable 63for constructorName.
  13. Let byteLength be elementSize × elementLength.
  14. IfIsSharedArrayBuffer(srcData) isfalse, then
    1. Let bufferConstructor be ? SpeciesConstructor(srcData,%ArrayBuffer%).
  15. Else,
    1. Let bufferConstructor be%ArrayBuffer%.
  16. If elementType is the same as srcType, then
    1. Let data be ? CloneArrayBuffer(srcData, srcByteOffset, byteLength, bufferConstructor).
  17. Else,
    1. Let data be ? AllocateArrayBuffer(bufferConstructor, byteLength).
    2. IfIsDetachedBuffer(srcData) istrue, throw aTypeErrorexception.
    3. If srcArray.[[ContentType]] ≠ O.[[ContentType]], throw aTypeErrorexception.
    4. Let srcByteIndex be srcByteOffset.
    5. Let targetByteIndex be 0.
    6. Let count be elementLength.
    7. Repeat, while count > 0,
      1. Let value beGetValueFromBuffer(srcData, srcByteIndex, srcType,true,Unordered).
      2. PerformSetValueInBuffer(data, targetByteIndex, elementType, value,true,Unordered).
      3. Set srcByteIndex to srcByteIndex + srcElementSize.
      4. Set targetByteIndex to targetByteIndex + elementSize.
      5. Set count to count - 1.
  18. Set O.[[ViewedArrayBuffer]] to data.
  19. Set O.[[ByteLength]] to byteLength.
  20. Set O.[[ByteOffset]] to 0.
  21. Set O.[[ArrayLength]] to elementLength.

23.2.5.1.3 InitializeTypedArrayFromArrayBuffer ( O, buffer, byteOffset, length )

The abstract operation InitializeTypedArrayFromArrayBuffer takes arguments O (a TypedArray object), buffer (an ArrayBuffer object), byteOffset (anECMAScript language value), and length (anECMAScript language value). It performs the following steps when called:

  1. Assert: O is an Object that has a [[TypedArrayName]] internal slot.
  2. Assert: buffer is an Object that has an [[ArrayBufferData]] internal slot.
  3. Let constructorName be the String value of O.[[TypedArrayName]].
  4. Let elementSize be the Element Size value specified inTable 63for constructorName.
  5. Let offset be ? ToIndex(byteOffset).
  6. If offsetmoduloelementSize ≠ 0, throw aRangeErrorexception.
  7. If length is notundefined, then
    1. Let newLength be ? ToIndex(length).
  8. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  9. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
  10. If length isundefined, then
    1. If bufferByteLengthmoduloelementSize ≠ 0, throw aRangeErrorexception.
    2. Let newByteLength be bufferByteLength - offset.
    3. If newByteLength < 0, throw aRangeErrorexception.
  11. Else,
    1. Let newByteLength be newLength × elementSize.
    2. If offset + newByteLength > bufferByteLength, throw aRangeErrorexception.
  12. Set O.[[ViewedArrayBuffer]] to buffer.
  13. Set O.[[ByteLength]] to newByteLength.
  14. Set O.[[ByteOffset]] to offset.
  15. Set O.[[ArrayLength]] to newByteLength / elementSize.

23.2.5.1.4 InitializeTypedArrayFromList ( O, values )

The abstract operation InitializeTypedArrayFromList takes arguments O (a TypedArray object) and values (aListof ECMAScript language values). It performs the following steps when called:

  1. Assert: O is an Object that has a [[TypedArrayName]] internal slot.
  2. Let len be the number of elements in values.
  3. Perform ? AllocateTypedArrayBuffer(O, len).
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be the first element of values and remove that element from values.
    3. Perform ? Set(O, Pk, kValue,true).
    4. Set k to k + 1.
  6. Assert: values is now an emptyList.

23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O, arrayLike )

The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray object) and arrayLike (an Object that is neither a TypedArray object nor an ArrayBuffer object). It performs the following steps when called:

  1. Assert: O is an Object that has a [[TypedArrayName]] internal slot.
  2. Let len be ? LengthOfArrayLike(arrayLike).
  3. Perform ? AllocateTypedArrayBuffer(O, len).
  4. Let k be 0.
  5. Repeat, while k < len,
    1. Let Pk be ! ToString(𝔽(k)).
    2. Let kValue be ? Get(arrayLike, Pk).
    3. Perform ? Set(O, Pk, kValue,true).
    4. Set k to k + 1.

23.2.5.1.6 AllocateTypedArrayBuffer ( O, length )

The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray object) and length (a non-negativeinteger). It allocates and associates an ArrayBuffer with O. It performs the following steps when called:

  1. Assert: O is an Object that has a [[ViewedArrayBuffer]] internal slot.
  2. Assert: O.[[ViewedArrayBuffer]] isundefined.
  3. Let constructorName be the String value of O.[[TypedArrayName]].
  4. Let elementSize be the Element Size value specified inTable 63for constructorName.
  5. Let byteLength be elementSize × length.
  6. Let data be ? AllocateArrayBuffer(%ArrayBuffer%, byteLength).
  7. Set O.[[ViewedArrayBuffer]] to data.
  8. Set O.[[ByteLength]] to byteLength.
  9. Set O.[[ByteOffset]] to 0.
  10. Set O.[[ArrayLength]] to length.
  11. Return O.

23.2.6 Properties of the TypedArray Constructors

Each TypedArrayconstructor:

  • has a [[Prototype]] internal slot whose value is%TypedArray%.
  • has a"name"property whose value is the String value of theconstructorname specified for it inTable 63.
  • has the following properties:

23.2.6.1 TypedArray.BYTES_PER_ELEMENT

The value of TypedArray.BYTES_PER_ELEMENT is the Element Size value specified inTable 63for TypedArray.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

23.2.6.2 TypedArray.prototype

The initial value of TypedArray.prototype is the corresponding TypedArray prototype intrinsic object (23.2.7).

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

23.2.7 Properties of the TypedArray Prototype Objects

Each TypedArray prototype object:

  • has a [[Prototype]] internal slot whose value is%TypedArray.prototype%.
  • is anordinary object.
  • does not have a [[ViewedArrayBuffer]] or any other of the internal slots that are specific to TypedArray instance objects.

23.2.7.1 TypedArray.prototype.BYTES_PER_ELEMENT

The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value specified inTable 63for TypedArray.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

23.2.7.2 TypedArray.prototype.constructor

The initial value of a TypedArray.prototype.constructor is the corresponding%TypedArray%intrinsic object.

23.2.8 Properties of TypedArray Instances

TypedArray instances areInteger-Indexed exotic objects. Each TypedArray instance inherits properties from the corresponding TypedArray prototype object. Each TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].

24 Keyed Collections

24.1 Map Objects

Map objects are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct key value may only occur in one key/value pair within the Map's collection. Distinct key values are discriminated using theSameValueZerocomparison algorithm.

Map object must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Map objects specification is only intended to describe the required observable semantics of Map objects. It is not intended to be a viable implementation model.

24.1.1 The Map Constructor

The Mapconstructor:

  • is %Map%.
  • is the initial value of the"Map"property of theglobal object.
  • creates and initializes a new Map object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Map behaviour must include a super call to the Mapconstructorto create and initialize the subclass instance with the internal state necessary to support the Map.prototype built-in methods.

24.1.1.1 Map ( [ iterable ] )

When the Map function is called with optional argument iterable, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let map be ? OrdinaryCreateFromConstructor(NewTarget,"%Map.prototype%", « [[MapData]] »).
  3. Set map.[[MapData]] to a new emptyList.
  4. If iterable is eitherundefinedornull, return map.
  5. Let adder be ? Get(map,"set").
  6. Return ? AddEntriesFromIterable(map, iterable, adder).
Note

If the parameter iterable is present, it is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

24.1.1.2 AddEntriesFromIterable ( target, iterable, adder )

The abstract operation AddEntriesFromIterable takes arguments target, iterable, and adder (afunction object). adder will be invoked, with target as the receiver. It performs the following steps when called:

  1. IfIsCallable(adder) isfalse, throw aTypeErrorexception.
  2. Assert: iterable is present, and is neitherundefinednornull.
  3. Let iteratorRecord be ? GetIterator(iterable).
  4. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return target.
    3. Let nextItem be ? IteratorValue(next).
    4. IfType(nextItem) is not Object, then
      1. Let error beThrowCompletion(a newly createdTypeErrorobject).
      2. Return ? IteratorClose(iteratorRecord, error).
    5. Let k beGet(nextItem,"0").
    6. If k is anabrupt completion, return ? IteratorClose(iteratorRecord, k).
    7. Let v beGet(nextItem,"1").
    8. If v is anabrupt completion, return ? IteratorClose(iteratorRecord, v).
    9. Let status beCall(adder, target, « k.[[Value]], v.[[Value]] »).
    10. If status is anabrupt completion, return ? IteratorClose(iteratorRecord, status).
Note

The parameter iterable is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a Map key and whose second element is the value to associate with that key.

24.1.2 Properties of the Map Constructor

The Mapconstructor:

24.1.2.1 Map.prototype

The initial value of Map.prototype is theMap prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

24.1.2.2 get Map [ @@species ]

Map[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

Methods that create derived collection objects should call@@speciesto determine theconstructorto use to create the derived objects. Subclassconstructormay over-ride@@speciesto change the defaultconstructorassignment.

24.1.3 Properties of the Map Prototype Object

The Map prototype object:

24.1.3.1 Map.prototype.clear ( )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. Set p.[[Key]] toempty.
    2. Set p.[[Value]] toempty.
  5. Returnundefined.
Note

The existing [[MapData]]Listis preserved because there may be existing Map Iterator objects that are suspended midway through iterating over thatList.

24.1.3.2 Map.prototype.constructor

The initial value of Map.prototype.constructor is%Map%.

24.1.3.3 Map.prototype.delete ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValueZero(p.[[Key]], key) istrue, then
      1. Set p.[[Key]] toempty.
      2. Set p.[[Value]] toempty.
      3. Returntrue.
  5. Returnfalse.
Note

The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.1.3.4 Map.prototype.entries ( )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Return ? CreateMapIterator(M,key+value).

24.1.3.5 Map.prototype.forEach ( callbackfn [ , thisArg ] )

When the forEach method is called with one or two arguments, the following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let entries be theListthat is M.[[MapData]].
  5. For eachRecord{ [[Key]], [[Value]] } e of entries, do
    1. If e.[[Key]] is notempty, then
      1. Perform ? Call(callbackfn, thisArg, « e.[[Value]], e.[[Key]], M »).
  6. Returnundefined.
Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each key/value pair present in the map object, in key insertion order. callbackfn is called only for keys of the map which actually exist; it is not called for keys that have been deleted from the map.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the value of the item, the key of the item, and the Map object being traversed.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to forEach begins are visited. A key will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Keys that are deleted after the call to forEach begins and before being visited are not visited unless the key is added again before the forEach call completes.

24.1.3.6 Map.prototype.get ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValueZero(p.[[Key]], key) istrue, return p.[[Value]].
  5. Returnundefined.

24.1.3.7 Map.prototype.has ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValueZero(p.[[Key]], key) istrue, returntrue.
  5. Returnfalse.

24.1.3.8 Map.prototype.keys ( )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Return ? CreateMapIterator(M,key).

24.1.3.9 Map.prototype.set ( key, value )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValueZero(p.[[Key]], key) istrue, then
      1. Set p.[[Value]] to value.
      2. Return M.
  5. If key is-0𝔽, set key to+0𝔽.
  6. Let p be theRecord{ [[Key]]: key, [[Value]]: value }.
  7. Append p as the last element of entries.
  8. Return M.

24.1.3.10 get Map.prototype.size

Map.prototype.size is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[MapData]]).
  3. Let entries be theListthat is M.[[MapData]].
  4. Let count be 0.
  5. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notempty, set count to count + 1.
  6. Return𝔽(count).

24.1.3.11 Map.prototype.values ( )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Return ? CreateMapIterator(M,value).

24.1.3.12 Map.prototype [ @@iterator ] ( )

The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the"entries"property.

24.1.3.13 Map.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Map".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.1.4 Properties of Map Instances

Map instances are ordinary objects that inherit properties from the Map prototype. Map instances also have a [[MapData]] internal slot.

24.1.5 Map Iterator Objects

A Map Iterator is an object, that represents a specific iteration over some specific Map instance object. There is not a namedconstructorfor Map Iterator objects. Instead, map iterator objects are created by calling certain methods of Map instance objects.

24.1.5.1 CreateMapIterator ( map, kind )

The abstract operation CreateMapIterator takes arguments map and kind. It is used to create iterator objects for Map methods that return such iterators. It performs the following steps when called:

  1. Assert: kind iskey+value,key, orvalue.
  2. Perform ? RequireInternalSlot(map, [[MapData]]).
  3. Let closure be a newAbstract Closurewith no parameters that captures map and kind and performs the following steps when called:
    1. Let entries be theListthat is map.[[MapData]].
    2. Let index be 0.
    3. Let numEntries be the number of elements of entries.
    4. Repeat, while index < numEntries,
      1. Let e be theRecord{ [[Key]], [[Value]] } that is the value of entries[index].
      2. Set index to index + 1.
      3. If e.[[Key]] is notempty, then
        1. If kind iskey, let result be e.[[Key]].
        2. Else if kind isvalue, let result be e.[[Value]].
        3. Else,
          1. Assert: kind iskey+value.
          2. Let result be ! CreateArrayFromListe.[[Key]], e.[[Value]] »).
        4. Perform ? Yield(result).
        5. NOTE: The number of elements in entries may have changed while execution of this abstract operation was paused byYield.
        6. Set numEntries to the number of elements of entries.
    5. Returnundefined.
  4. Return ! CreateIteratorFromClosure(closure,"%MapIteratorPrototype%",%MapIteratorPrototype%).

24.1.5.2 The %MapIteratorPrototype% Object

The %MapIteratorPrototype% object:

  • has properties that are inherited by all Map Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has the following properties:

24.1.5.2.1 %MapIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(thisvalue,empty,"%MapIteratorPrototype%").

24.1.5.2.2 %MapIteratorPrototype% [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Map Iterator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.2 Set Objects

Set objects are collections of ECMAScript language values. A distinct value may only occur once as an element of a Set's collection. Distinct values are discriminated using theSameValueZerocomparison algorithm.

Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structures used in this Set objects specification is only intended to describe the required observable semantics of Set objects. It is not intended to be a viable implementation model.

24.2.1 The Set Constructor

The Setconstructor:

  • is %Set%.
  • is the initial value of the"Set"property of theglobal object.
  • creates and initializes a new Set object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Set behaviour must include a super call to the Setconstructorto create and initialize the subclass instance with the internal state necessary to support the Set.prototype built-in methods.

24.2.1.1 Set ( [ iterable ] )

When the Set function is called with optional argument iterable, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let set be ? OrdinaryCreateFromConstructor(NewTarget,"%Set.prototype%", « [[SetData]] »).
  3. Set set.[[SetData]] to a new emptyList.
  4. If iterable is eitherundefinedornull, return set.
  5. Let adder be ? Get(set,"add").
  6. IfIsCallable(adder) isfalse, throw aTypeErrorexception.
  7. Let iteratorRecord be ? GetIterator(iterable).
  8. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return set.
    3. Let nextValue be ? IteratorValue(next).
    4. Let status beCall(adder, set, « nextValue »).
    5. If status is anabrupt completion, return ? IteratorClose(iteratorRecord, status).

24.2.2 Properties of the Set Constructor

The Setconstructor:

24.2.2.1 Set.prototype

The initial value of Set.prototype is theSet prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

24.2.2.2 get Set [ @@species ]

Set[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

Methods that create derived collection objects should call@@speciesto determine theconstructorto use to create the derived objects. Subclassconstructormay over-ride@@speciesto change the defaultconstructorassignment.

24.2.3 Properties of the Set Prototype Object

The Set prototype object:

24.2.3.1 Set.prototype.add ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let entries be theListthat is S.[[SetData]].
  4. For each element e of entries, do
    1. If e is notemptyandSameValueZero(e, value) istrue, then
      1. Return S.
  5. If value is-0𝔽, set value to+0𝔽.
  6. Append value as the last element of entries.
  7. Return S.

24.2.3.2 Set.prototype.clear ( )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let entries be theListthat is S.[[SetData]].
  4. For each element e of entries, do
    1. Replace the element of entries whose value is e with an element whose value isempty.
  5. Returnundefined.
Note

The existing [[SetData]]Listis preserved because there may be existing Set Iterator objects that are suspended midway through iterating over thatList.

24.2.3.3 Set.prototype.constructor

The initial value of Set.prototype.constructor is%Set%.

24.2.3.4 Set.prototype.delete ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let entries be theListthat is S.[[SetData]].
  4. For each element e of entries, do
    1. If e is notemptyandSameValueZero(e, value) istrue, then
      1. Replace the element of entries whose value is e with an element whose value isempty.
      2. Returntrue.
  5. Returnfalse.
Note

The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.2.3.5 Set.prototype.entries ( )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateSetIterator(S,key+value).
Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

24.2.3.6 Set.prototype.forEach ( callbackfn [ , thisArg ] )

When the forEach method is called with one or two arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. IfIsCallable(callbackfn) isfalse, throw aTypeErrorexception.
  4. Let entries be theListthat is S.[[SetData]].
  5. For each element e of entries, do
    1. If e is notempty, then
      1. Perform ? Call(callbackfn, thisArg, « e, e, S »).
  6. Returnundefined.
Note

callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each value present in the set object, in value insertion order. callbackfn is called only for values of the Set which actually exist; it is not called for keys that have been deleted from the set.

If a thisArg parameter is provided, it will be used as thethisvalue for each invocation of callbackfn. If it is not provided,undefinedis used instead.

callbackfn is called with three arguments: the first two arguments are a value contained in the Set. The same value is passed for both arguments. The Set object being traversed is passed as the third argument.

The callbackfn is called with three arguments to be consistent with the call back functions used by forEach methods for Map and Array. For Sets, each item value is considered to be both the key and the value.

forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to callbackfn.

Each value is normally visited only once. However, a value will be revisited if it is deleted after it has been visited and then re-added before the forEach call completes. Values that are deleted after the call to forEach begins and before being visited are not visited unless the value is added again before the forEach call completes. New values added after the call to forEach begins are visited.

24.2.3.7 Set.prototype.has ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let entries be theListthat is S.[[SetData]].
  4. For each element e of entries, do
    1. If e is notemptyandSameValueZero(e, value) istrue, returntrue.
  5. Returnfalse.

24.2.3.8 Set.prototype.keys ( )

The initial value of the"keys"property is the samefunction objectas the initial value of the"values"property.

Note

For iteration purposes, a Set appears similar to a Map where each entry has the same value for its key and value.

24.2.3.9 get Set.prototype.size

Set.prototype.size is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[SetData]]).
  3. Let entries be theListthat is S.[[SetData]].
  4. Let count be 0.
  5. For each element e of entries, do
    1. If e is notempty, set count to count + 1.
  6. Return𝔽(count).

24.2.3.10 Set.prototype.values ( )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateSetIterator(S,value).

24.2.3.11 Set.prototype [ @@iterator ] ( )

The initial value of the@@iteratorproperty is the samefunction objectas the initial value of the"values"property.

24.2.3.12 Set.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Set".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.2.4 Properties of Set Instances

Set instances are ordinary objects that inherit properties from the Set prototype. Set instances also have a [[SetData]] internal slot.

24.2.5 Set Iterator Objects

A Set Iterator is anordinary object, with the structure defined below, that represents a specific iteration over some specific Set instance object. There is not a namedconstructorfor Set Iterator objects. Instead, set iterator objects are created by calling certain methods of Set instance objects.

24.2.5.1 CreateSetIterator ( set, kind )

The abstract operation CreateSetIterator takes arguments set and kind. It is used to create iterator objects for Set methods that return such iterators. It performs the following steps when called:

  1. Assert: kind iskey+valueorvalue.
  2. Perform ? RequireInternalSlot(set, [[SetData]]).
  3. Let closure be a newAbstract Closurewith no parameters that captures set and kind and performs the following steps when called:
    1. Let index be 0.
    2. Let entries be theListthat is set.[[SetData]].
    3. Let numEntries be the number of elements of entries.
    4. Repeat, while index < numEntries,
      1. Let e be entries[index].
      2. Set index to index + 1.
      3. If e is notempty, then
        1. If kind iskey+value, then
          1. Perform ? Yield(!CreateArrayFromListe, e »)).
        2. Else,
          1. Assert: kind isvalue.
          2. Perform ? Yield(e).
        3. NOTE: The number of elements in entries may have changed while execution of this abstract operation was paused byYield.
        4. Set numEntries to the number of elements of entries.
    5. Returnundefined.
  4. Return ! CreateIteratorFromClosure(closure,"%SetIteratorPrototype%",%SetIteratorPrototype%).

24.2.5.2 The %SetIteratorPrototype% Object

The %SetIteratorPrototype% object:

  • has properties that are inherited by all Set Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has the following properties:

24.2.5.2.1 %SetIteratorPrototype%.next ( )

  1. Return ? GeneratorResume(thisvalue,empty,"%SetIteratorPrototype%").

24.2.5.2.2 %SetIteratorPrototype% [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Set Iterator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.3 WeakMap Objects

WeakMap objects are collections of key/value pairs where the keys are objects and values may be arbitrary ECMAScript language values. A WeakMap may be queried to see if it contains a key/value pair with a specific key, but no mechanism is provided for enumerating the objects it holds as keys. In certain conditions, objects which are notliveare removed as WeakMap keys, as described in9.10.3.

An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that does not require the observer to present the observed key.

WeakMap objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of key/value pairs in the collection. The data structure used in this WeakMap objects specification are only intended to describe the required observable semantics of WeakMap objects. It is not intended to be a viable implementation model.

Note

WeakMap and WeakSets are intended to provide mechanisms for dynamically associating state with an object in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet, the object otherwise became inaccessible and subject to resource reclamation by the implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted per-object mapping of weak map instances to keys. Alternatively each weak map may internally store its key to value mappings but this approach requires coordination between the WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism that may be useful to implementations of WeakMap and WeakSets:

Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.

Alexandra Barros, Roberto Ierusalimschy, Eliminating Cycles in Weak Tables. Journal of Universal Computer Science - J.UCS, vol. 14, no. 21, pp. 3481-3497, 2008, http://www.jucs.org/jucs_14_21/eliminating_cycles_in_weak

24.3.1 The WeakMap Constructor

The WeakMapconstructor:

  • is %WeakMap%.
  • is the initial value of the"WeakMap"property of theglobal object.
  • creates and initializes a new WeakMap object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakMap behaviour must include a super call to the WeakMapconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakMap.prototype built-in methods.

24.3.1.1 WeakMap ( [ iterable ] )

When the WeakMap function is called with optional argument iterable, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let map be ? OrdinaryCreateFromConstructor(NewTarget,"%WeakMap.prototype%", « [[WeakMapData]] »).
  3. Set map.[[WeakMapData]] to a new emptyList.
  4. If iterable is eitherundefinedornull, return map.
  5. Let adder be ? Get(map,"set").
  6. Return ? AddEntriesFromIterable(map, iterable, adder).
Note

If the parameter iterable is present, it is expected to be an object that implements an@@iteratormethod that returns an iterator object that produces a two elementarray-like objectwhose first element is a value that will be used as a WeakMap key and whose second element is the value to associate with that key.

24.3.2 Properties of the WeakMap Constructor

The WeakMapconstructor:

24.3.2.1 WeakMap.prototype

The initial value of WeakMap.prototype is theWeakMap prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

24.3.3 Properties of the WeakMap Prototype Object

The WeakMap prototype object:

24.3.3.1 WeakMap.prototype.constructor

The initial value of WeakMap.prototype.constructor is%WeakMap%.

24.3.3.2 WeakMap.prototype.delete ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be theListthat is M.[[WeakMapData]].
  4. IfType(key) is not Object, returnfalse.
  5. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, then
      1. Set p.[[Key]] toempty.
      2. Set p.[[Value]] toempty.
      3. Returntrue.
  6. Returnfalse.
Note

The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.3.3.3 WeakMap.prototype.get ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be theListthat is M.[[WeakMapData]].
  4. IfType(key) is not Object, returnundefined.
  5. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, return p.[[Value]].
  6. Returnundefined.

24.3.3.4 WeakMap.prototype.has ( key )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be theListthat is M.[[WeakMapData]].
  4. IfType(key) is not Object, returnfalse.
  5. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, returntrue.
  6. Returnfalse.

24.3.3.5 WeakMap.prototype.set ( key, value )

The following steps are taken:

  1. Let M be thethisvalue.
  2. Perform ? RequireInternalSlot(M, [[WeakMapData]]).
  3. Let entries be theListthat is M.[[WeakMapData]].
  4. IfType(key) is not Object, throw aTypeErrorexception.
  5. For eachRecord{ [[Key]], [[Value]] } p of entries, do
    1. If p.[[Key]] is notemptyandSameValue(p.[[Key]], key) istrue, then
      1. Set p.[[Value]] to value.
      2. Return M.
  6. Let p be theRecord{ [[Key]]: key, [[Value]]: value }.
  7. Append p as the last element of entries.
  8. Return M.

24.3.3.6 WeakMap.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"WeakMap".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.3.4 Properties of WeakMap Instances

WeakMap instances are ordinary objects that inherit properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.

24.4 WeakSet Objects

WeakSet objects are collections of objects. A distinct object may only occur once as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific object, but no mechanism is provided for enumerating the objects it holds. In certain conditions, objects which are notliveare removed as WeakSet elements, as described in9.10.3.

An implementation may impose an arbitrarily determined latency between the time an object contained in a WeakSet becomes inaccessible and the time when the object is removed from the WeakSet. If this latency was observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution. For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a particular object that does not require the observer to present the observed object.

WeakSet objects must be implemented using either hash tables or other mechanisms that, on average, provide access times that are sublinear on the number of elements in the collection. The data structure used in this WeakSet objects specification is only intended to describe the required observable semantics of WeakSet objects. It is not intended to be a viable implementation model.

Note

See the NOTE in24.3.

24.4.1 The WeakSet Constructor

The WeakSetconstructor:

  • is %WeakSet%.
  • is the initial value of the"WeakSet"property of theglobal object.
  • creates and initializes a new WeakSet object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakSet behaviour must include a super call to the WeakSetconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakSet.prototype built-in methods.

24.4.1.1 WeakSet ( [ iterable ] )

When the WeakSet function is called with optional argument iterable, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let set be ? OrdinaryCreateFromConstructor(NewTarget,"%WeakSet.prototype%", « [[WeakSetData]] »).
  3. Set set.[[WeakSetData]] to a new emptyList.
  4. If iterable is eitherundefinedornull, return set.
  5. Let adder be ? Get(set,"add").
  6. IfIsCallable(adder) isfalse, throw aTypeErrorexception.
  7. Let iteratorRecord be ? GetIterator(iterable).
  8. Repeat,
    1. Let next be ? IteratorStep(iteratorRecord).
    2. If next isfalse, return set.
    3. Let nextValue be ? IteratorValue(next).
    4. Let status beCall(adder, set, « nextValue »).
    5. If status is anabrupt completion, return ? IteratorClose(iteratorRecord, status).

24.4.2 Properties of the WeakSet Constructor

The WeakSetconstructor:

24.4.2.1 WeakSet.prototype

The initial value of WeakSet.prototype is theWeakSet prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

24.4.3 Properties of the WeakSet Prototype Object

The WeakSet prototype object:

24.4.3.1 WeakSet.prototype.add ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. IfType(value) is not Object, throw aTypeErrorexception.
  4. Let entries be theListthat is S.[[WeakSetData]].
  5. For each element e of entries, do
    1. If e is notemptyandSameValue(e, value) istrue, then
      1. Return S.
  6. Append value as the last element of entries.
  7. Return S.

24.4.3.2 WeakSet.prototype.constructor

The initial value of WeakSet.prototype.constructor is the%WeakSet%intrinsic object.

24.4.3.3 WeakSet.prototype.delete ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. IfType(value) is not Object, returnfalse.
  4. Let entries be theListthat is S.[[WeakSetData]].
  5. For each element e of entries, do
    1. If e is notemptyandSameValue(e, value) istrue, then
      1. Replace the element of entries whose value is e with an element whose value isempty.
      2. Returntrue.
  6. Returnfalse.
Note

The valueemptyis used as a specification device to indicate that an entry has been deleted. Actual implementations may take other actions such as physically removing the entry from internal data structures.

24.4.3.4 WeakSet.prototype.has ( value )

The following steps are taken:

  1. Let S be thethisvalue.
  2. Perform ? RequireInternalSlot(S, [[WeakSetData]]).
  3. Let entries be theListthat is S.[[WeakSetData]].
  4. IfType(value) is not Object, returnfalse.
  5. For each element e of entries, do
    1. If e is notemptyandSameValue(e, value) istrue, returntrue.
  6. Returnfalse.

24.4.3.5 WeakSet.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"WeakSet".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

24.4.4 Properties of WeakSet Instances

WeakSet instances are ordinary objects that inherit properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.

25 Structured Data

25.1 ArrayBuffer Objects

25.1.1 Notation

The descriptions below in this section,25.4, and29use the read-modify-write modification function internal data structure.

A read-modify-write modification function is a mathematical function that is notationally represented as an abstract closure that takes two Lists of byte values as arguments and returns aListof byte values. These abstract closures satisfy all of the following properties:

  • They perform all their algorithm steps atomically.
  • Their individual algorithm steps are not observable.
Note

To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure, mathematical function, the following editorial conventions are recommended:

  • They do not access, directly or transitively via invokedabstract operationsand abstract closures, any language or specification values except their parameters and captured values.
  • They do notreturn completionvalues.

25.1.2 Abstract Operations For ArrayBuffer Objects

25.1.2.1 AllocateArrayBuffer ( constructor, byteLength )

The abstract operation AllocateArrayBuffer takes arguments constructor and byteLength (a non-negativeinteger). It is used to create an ArrayBuffer object. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(constructor,"%ArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] »).
  2. Let block be ? CreateByteDataBlock(byteLength).
  3. Set obj.[[ArrayBufferData]] to block.
  4. Set obj.[[ArrayBufferByteLength]] to byteLength.
  5. Return obj.

25.1.2.2 IsDetachedBuffer ( arrayBuffer )

The abstract operation IsDetachedBuffer takes argument arrayBuffer. It performs the following steps when called:

  1. Assert:Type(arrayBuffer) is Object and arrayBuffer has an [[ArrayBufferData]] internal slot.
  2. If arrayBuffer.[[ArrayBufferData]] isnull, returntrue.
  3. Returnfalse.

25.1.2.3 DetachArrayBuffer ( arrayBuffer [ , key ] )

The abstract operation DetachArrayBuffer takes argument arrayBuffer and optional argument key. It performs the following steps when called:

  1. Assert:Type(arrayBuffer) is Object and arrayBuffer has [[ArrayBufferData]], [[ArrayBufferByteLength]], and [[ArrayBufferDetachKey]] internal slots.
  2. Assert:IsSharedArrayBuffer(arrayBuffer) isfalse.
  3. If key is not present, set key toundefined.
  4. IfSameValue(arrayBuffer.[[ArrayBufferDetachKey]], key) isfalse, throw aTypeErrorexception.
  5. Set arrayBuffer.[[ArrayBufferData]] tonull.
  6. Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
  7. ReturnNormalCompletion(null).
Note

Detaching an ArrayBuffer instance disassociates theData Blockused as its backing store from the instance and sets the byte length of the buffer to 0. No operations defined by this specification use the DetachArrayBuffer abstract operation. However, an ECMAScripthostor implementation may define such operations.

25.1.2.4 CloneArrayBuffer ( srcBuffer, srcByteOffset, srcLength, cloneConstructor )

The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer object), srcByteOffset (a non-negativeinteger), srcLength (a non-negativeinteger), and cloneConstructor (aconstructor). It creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range starting at srcByteOffset and continuing for srcLength bytes. It performs the following steps when called:

  1. Assert:Type(srcBuffer) is Object and srcBuffer has an [[ArrayBufferData]] internal slot.
  2. Assert:IsConstructor(cloneConstructor) istrue.
  3. Let targetBuffer be ? AllocateArrayBuffer(cloneConstructor, srcLength).
  4. IfIsDetachedBuffer(srcBuffer) istrue, throw aTypeErrorexception.
  5. Let srcBlock be srcBuffer.[[ArrayBufferData]].
  6. Let targetBlock be targetBuffer.[[ArrayBufferData]].
  7. PerformCopyDataBlockBytes(targetBlock, 0, srcBlock, srcByteOffset, srcLength).
  8. Return targetBuffer.

25.1.2.5 IsUnsignedElementType ( type )

The abstract operation IsUnsignedElementType takes argument type. It verifies if the argument type is an unsignedTypedArray element type. It performs the following steps when called:

  1. If type isUint8,Uint8C,Uint16,Uint32, orBigUint64, returntrue.
  2. Returnfalse.

25.1.2.6 IsUnclampedIntegerElementType ( type )

The abstract operation IsUnclampedIntegerElementType takes argument type. It verifies if the argument type is anIntegerTypedArray element typenot includingUint8C. It performs the following steps when called:

  1. If type isInt8,Uint8,Int16,Uint16,Int32, orUint32, returntrue.
  2. Returnfalse.

25.1.2.7 IsBigIntElementType ( type )

The abstract operation IsBigIntElementType takes argument type. It verifies if the argument type is a BigIntTypedArray element type. It performs the following steps when called:

  1. If type isBigUint64orBigInt64, returntrue.
  2. Returnfalse.

25.1.2.8 IsNoTearConfiguration ( type, order )

The abstract operation IsNoTearConfiguration takes arguments type and order. It performs the following steps when called:

  1. If ! IsUnclampedIntegerElementType(type) istrue, returntrue.
  2. If ! IsBigIntElementType(type) istrueand order is notInitorUnordered, returntrue.
  3. Returnfalse.

25.1.2.9 RawBytesToNumeric ( type, rawBytes, isLittleEndian )

The abstract operation RawBytesToNumeric takes arguments type (aTypedArray element type), rawBytes (aList), and isLittleEndian (a Boolean). It performs the following steps when called:

  1. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  2. If isLittleEndian isfalse, reverse the order of the elements of rawBytes.
  3. If type isFloat32, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of anIEEE 754-2019binary32 value.
    2. If value is anIEEE 754-2019binary32 NaN value, return theNaNNumber value.
    3. Return theNumber valuethat corresponds to value.
  4. If type isFloat64, then
    1. Let value be the byte elements of rawBytes concatenated and interpreted as a little-endian bit string encoding of anIEEE 754-2019binary64 value.
    2. If value is anIEEE 754-2019binary64 NaN value, return theNaNNumber value.
    3. Return theNumber valuethat corresponds to value.
  5. If ! IsUnsignedElementType(type) istrue, then
    1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of an unsigned little-endian binary number.
  6. Else,
    1. Let intValue be the byte elements of rawBytes concatenated and interpreted as a bit string encoding of a binary little-endian two's complement number of bit length elementSize × 8.
  7. If ! IsBigIntElementType(type) istrue, return the BigInt value that corresponds to intValue.
  8. Otherwise, return theNumber valuethat corresponds to intValue.

25.1.2.10 GetValueFromBuffer ( arrayBuffer, byteIndex, type, isTypedArray, order [ , isLittleEndian ] )

The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negativeinteger), type (aTypedArray element type), isTypedArray (a Boolean), and order (eitherSeqCstorUnordered) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

  1. Assert:IsDetachedBuffer(arrayBuffer) isfalse.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Let block be arrayBuffer.[[ArrayBufferData]].
  4. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  5. IfIsSharedArrayBuffer(arrayBuffer) istrue, then
    1. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
    2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
    3. If isTypedArray istrueandIsNoTearConfiguration(type, order) istrue, let noTear betrue; otherwise let noTear befalse.
    4. Let rawValue be aListof length elementSize whose elements are nondeterministically chosen byte values.
    5. NOTE: In implementations, rawValue is the result of a non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
    6. Let readEvent beReadSharedMemory{ [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
    7. Append readEvent to eventList.
    8. AppendChosen Value Record{ [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
  6. Else, let rawValue be aListwhose elements are bytes from block at indices byteIndex (inclusive) through byteIndex + elementSize (exclusive).
  7. Assert: The number of elements in rawValue is elementSize.
  8. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  9. ReturnRawBytesToNumeric(type, rawValue, isLittleEndian).

25.1.2.11 NumericToRawBytes ( type, value, isLittleEndian )

The abstract operation NumericToRawBytes takes arguments type (aTypedArray element type), value (a BigInt or a Number), and isLittleEndian (a Boolean). It performs the following steps when called:

  1. If type isFloat32, then
    1. Let rawBytes be aListwhose elements are the 4 bytes that are the result of converting value toIEEE 754-2019binary32 format using roundTiesToEven mode. If isLittleEndian isfalse, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN, rawBytes may be set to any implementation chosenIEEE 754-2019binary32 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaNvalue.
  2. Else if type isFloat64, then
    1. Let rawBytes be aListwhose elements are the 8 bytes that are theIEEE 754-2019binary64 format encoding of value. If isLittleEndian isfalse, the bytes are arranged in big endian order. Otherwise, the bytes are arranged in little endian order. If value isNaN, rawBytes may be set to any implementation chosenIEEE 754-2019binary64 format Not-a-Number encoding. An implementation must always choose the same encoding for each implementation distinguishableNaNvalue.
  3. Else,
    1. Let n be the Element Size value specified inTable 63for Element Type type.
    2. Let convOp be the abstract operation named in the Conversion Operation column inTable 63for Element Type type.
    3. Let intValue be(convOp(value)).
    4. If intValue ≥ 0, then
      1. Let rawBytes be aListwhose elements are the n-byte binary encoding of intValue. If isLittleEndian isfalse, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
    5. Else,
      1. Let rawBytes be aListwhose elements are the n-byte binary two's complement encoding of intValue. If isLittleEndian isfalse, the bytes are ordered in big endian order. Otherwise, the bytes are ordered in little endian order.
  4. Return rawBytes.

25.1.2.12 SetValueInBuffer ( arrayBuffer, byteIndex, type, value, isTypedArray, order [ , isLittleEndian ] )

The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negativeinteger), type (aTypedArray element type), value (a Number or a BigInt), isTypedArray (a Boolean), and order (one ofSeqCst,Unordered, orInit) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

  1. Assert:IsDetachedBuffer(arrayBuffer) isfalse.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert:Type(value) is BigInt if ! IsBigIntElementType(type) istrue; otherwise,Type(value) is Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  6. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  7. Let rawBytes beNumericToRawBytes(type, value, isLittleEndian).
  8. IfIsSharedArrayBuffer(arrayBuffer) istrue, then
    1. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
    2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
    3. If isTypedArray istrueandIsNoTearConfiguration(type, order) istrue, let noTear betrue; otherwise let noTear befalse.
    4. AppendWriteSharedMemory{ [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventList.
  9. Else, store the individual bytes of rawBytes into block, starting at block[byteIndex].
  10. ReturnNormalCompletion(undefined).

25.1.2.13 GetModifySetValueInBuffer ( arrayBuffer, byteIndex, type, value, op [ , isLittleEndian ] )

The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer object or a SharedArrayBuffer object), byteIndex (a non-negativeinteger), type (aTypedArray element type), value (a Number or a BigInt), and op (aread-modify-write modification function) and optional argument isLittleEndian (a Boolean). It performs the following steps when called:

  1. Assert:IsDetachedBuffer(arrayBuffer) isfalse.
  2. Assert: There are sufficient bytes in arrayBuffer starting at byteIndex to represent a value of type.
  3. Assert:Type(value) is BigInt if ! IsBigIntElementType(type) istrue; otherwise,Type(value) is Number.
  4. Let block be arrayBuffer.[[ArrayBufferData]].
  5. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  6. If isLittleEndian is not present, set isLittleEndian to the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  7. Let rawBytes beNumericToRawBytes(type, value, isLittleEndian).
  8. IfIsSharedArrayBuffer(arrayBuffer) istrue, then
    1. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
    2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
    3. Let rawBytesRead be aListof length elementSize whose elements are nondeterministically chosen byte values.
    4. NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
    5. Let rmwEvent beReadModifyWriteSharedMemory{ [[Order]]:SeqCst, [[NoTear]]:true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
    6. Append rmwEvent to eventList.
    7. AppendChosen Value Record{ [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
  9. Else,
    1. Let rawBytesRead be aListof length elementSize whose elements are the sequence of elementSize bytes starting with block[byteIndex].
    2. Let rawBytesModified be op(rawBytesRead, rawBytes).
    3. Store the individual bytes of rawBytesModified into block, starting at block[byteIndex].
  10. ReturnRawBytesToNumeric(type, rawBytesRead, isLittleEndian).

25.1.3 The ArrayBuffer Constructor

The ArrayBufferconstructor:

  • is %ArrayBuffer%.
  • is the initial value of the"ArrayBuffer"property of theglobal object.
  • creates and initializes a new ArrayBuffer object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified ArrayBuffer behaviour must include a super call to the ArrayBufferconstructorto create and initialize subclass instances with the internal state necessary to support the ArrayBuffer.prototype built-in methods.

25.1.3.1 ArrayBuffer ( length )

When the ArrayBuffer function is called with argument length, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let byteLength be ? ToIndex(length).
  3. Return ? AllocateArrayBuffer(NewTarget, byteLength).

25.1.4 Properties of the ArrayBuffer Constructor

The ArrayBufferconstructor:

25.1.4.1 ArrayBuffer.isView ( arg )

The isView function takes one argument arg, and performs the following steps:

  1. IfType(arg) is not Object, returnfalse.
  2. If arg has a [[ViewedArrayBuffer]] internal slot, returntrue.
  3. Returnfalse.

25.1.4.2 ArrayBuffer.prototype

The initial value of ArrayBuffer.prototype is theArrayBuffer prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

25.1.4.3 get ArrayBuffer [ @@species ]

ArrayBuffer[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

ArrayBuffer prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.

25.1.5 Properties of the ArrayBuffer Prototype Object

The ArrayBuffer prototype object:

  • is %ArrayBuffer.prototype%.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is anordinary object.
  • does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

25.1.5.1 get ArrayBuffer.prototype.byteLength

ArrayBuffer.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. IfIsSharedArrayBuffer(O) istrue, throw aTypeErrorexception.
  4. IfIsDetachedBuffer(O) istrue, return+0𝔽.
  5. Let length be O.[[ArrayBufferByteLength]].
  6. Return𝔽(length).

25.1.5.2 ArrayBuffer.prototype.constructor

The initial value of ArrayBuffer.prototype.constructor is%ArrayBuffer%.

25.1.5.3 ArrayBuffer.prototype.slice ( start, end )

The following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. IfIsSharedArrayBuffer(O) istrue, throw aTypeErrorexception.
  4. IfIsDetachedBuffer(O) istrue, throw aTypeErrorexception.
  5. Let len be O.[[ArrayBufferByteLength]].
  6. Let relativeStart be ? ToIntegerOrInfinity(start).
  7. If relativeStart is -∞, let first be 0.
  8. Else if relativeStart < 0, let first bemax(len + relativeStart, 0).
  9. Else, let first bemin(relativeStart, len).
  10. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  11. If relativeEnd is -∞, let final be 0.
  12. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  13. Else, let final bemin(relativeEnd, len).
  14. Let newLen bemax(final - first, 0).
  15. Let ctor be ? SpeciesConstructor(O,%ArrayBuffer%).
  16. Let new be ? Construct(ctor, «𝔽(newLen) »).
  17. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
  18. IfIsSharedArrayBuffer(new) istrue, throw aTypeErrorexception.
  19. IfIsDetachedBuffer(new) istrue, throw aTypeErrorexception.
  20. IfSameValue(new, O) istrue, throw aTypeErrorexception.
  21. If new.[[ArrayBufferByteLength]] < newLen, throw aTypeErrorexception.
  22. NOTE: Side-effects of the above steps may have detached O.
  23. IfIsDetachedBuffer(O) istrue, throw aTypeErrorexception.
  24. Let fromBuf be O.[[ArrayBufferData]].
  25. Let toBuf be new.[[ArrayBufferData]].
  26. PerformCopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
  27. Return new.

25.1.5.4 ArrayBuffer.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"ArrayBuffer".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

25.1.6 Properties of ArrayBuffer Instances

ArrayBuffer instances inherit properties from theArrayBuffer prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot.

ArrayBuffer instances whose [[ArrayBufferData]] isnullare considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance will fail.

ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other thanundefinedneed to have allDetachArrayBuffercalls passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only ever set by certain embedding environments, not by algorithms in this specification.

25.2 SharedArrayBuffer Objects

25.2.1 Abstract Operations for SharedArrayBuffer Objects

25.2.1.1 AllocateSharedArrayBuffer ( constructor, byteLength )

The abstract operation AllocateSharedArrayBuffer takes arguments constructor and byteLength (a non-negativeinteger). It is used to create a SharedArrayBuffer object. It performs the following steps when called:

  1. Let obj be ? OrdinaryCreateFromConstructor(constructor,"%SharedArrayBuffer.prototype%", « [[ArrayBufferData]], [[ArrayBufferByteLength]] »).
  2. Let block be ? CreateSharedByteDataBlock(byteLength).
  3. Set obj.[[ArrayBufferData]] to block.
  4. Set obj.[[ArrayBufferByteLength]] to byteLength.
  5. Return obj.

25.2.1.2 IsSharedArrayBuffer ( obj )

The abstract operation IsSharedArrayBuffer takes argument obj. It tests whether an object is an ArrayBuffer, a SharedArrayBuffer, or a subtype of either. It performs the following steps when called:

  1. Assert:Type(obj) is Object and obj has an [[ArrayBufferData]] internal slot.
  2. Let bufferData be obj.[[ArrayBufferData]].
  3. If bufferData isnull, returnfalse.
  4. If bufferData is aData Block, returnfalse.
  5. Assert: bufferData is aShared Data Block.
  6. Returntrue.

25.2.2 The SharedArrayBuffer Constructor

The SharedArrayBufferconstructor:

  • is %SharedArrayBuffer%.
  • is the initial value of the"SharedArrayBuffer"property of theglobal object, if that property is present (see below).
  • creates and initializes a new SharedArrayBuffer object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBufferconstructorto create and initialize subclass instances with the internal state necessary to support the SharedArrayBuffer.prototype built-in methods.

Whenever ahostdoes not provide concurrent access to SharedArrayBuffer objects it may omit the"SharedArrayBuffer"property of theglobal object.

Note

Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its internal [[ArrayBufferData]] slot is nevernull.

25.2.2.1 SharedArrayBuffer ( length )

When the SharedArrayBuffer function is called with argument length, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Let byteLength be ? ToIndex(length).
  3. Return ? AllocateSharedArrayBuffer(NewTarget, byteLength).

25.2.3 Properties of the SharedArrayBuffer Constructor

The SharedArrayBufferconstructor:

25.2.3.1 SharedArrayBuffer.prototype

The initial value of SharedArrayBuffer.prototype is theSharedArrayBuffer prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

25.2.3.2 get SharedArrayBuffer [ @@species ]

SharedArrayBuffer[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

25.2.4 Properties of the SharedArrayBuffer Prototype Object

The SharedArrayBuffer prototype object:

  • is %SharedArrayBuffer.prototype%.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is anordinary object.
  • does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.

25.2.4.1 get SharedArrayBuffer.prototype.byteLength

SharedArrayBuffer.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. IfIsSharedArrayBuffer(O) isfalse, throw aTypeErrorexception.
  4. Let length be O.[[ArrayBufferByteLength]].
  5. Return𝔽(length).

25.2.4.2 SharedArrayBuffer.prototype.constructor

The initial value of SharedArrayBuffer.prototype.constructor is%SharedArrayBuffer%.

25.2.4.3 SharedArrayBuffer.prototype.slice ( start, end )

The following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[ArrayBufferData]]).
  3. IfIsSharedArrayBuffer(O) isfalse, throw aTypeErrorexception.
  4. Let len be O.[[ArrayBufferByteLength]].
  5. Let relativeStart be ? ToIntegerOrInfinity(start).
  6. If relativeStart is -∞, let first be 0.
  7. Else if relativeStart < 0, let first bemax(len + relativeStart, 0).
  8. Else, let first bemin(relativeStart, len).
  9. If end isundefined, let relativeEnd be len; else let relativeEnd be ? ToIntegerOrInfinity(end).
  10. If relativeEnd is -∞, let final be 0.
  11. Else if relativeEnd < 0, let final bemax(len + relativeEnd, 0).
  12. Else, let final bemin(relativeEnd, len).
  13. Let newLen bemax(final - first, 0).
  14. Let ctor be ? SpeciesConstructor(O,%SharedArrayBuffer%).
  15. Let new be ? Construct(ctor, «𝔽(newLen) »).
  16. Perform ? RequireInternalSlot(new, [[ArrayBufferData]]).
  17. IfIsSharedArrayBuffer(new) isfalse, throw aTypeErrorexception.
  18. If new.[[ArrayBufferData]] and O.[[ArrayBufferData]] are the sameShared Data Blockvalues, throw aTypeErrorexception.
  19. If new.[[ArrayBufferByteLength]] < newLen, throw aTypeErrorexception.
  20. Let fromBuf be O.[[ArrayBufferData]].
  21. Let toBuf be new.[[ArrayBufferData]].
  22. PerformCopyDataBlockBytes(toBuf, 0, fromBuf, first, newLen).
  23. Return new.

25.2.4.4 SharedArrayBuffer.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"SharedArrayBuffer".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

25.2.5 Properties of SharedArrayBuffer Instances

SharedArrayBuffer instances inherit properties from theSharedArrayBuffer prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot and an [[ArrayBufferByteLength]] internal slot.

Note

SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.

25.3 DataView Objects

25.3.1 Abstract Operations For DataView Objects

25.3.1.1 GetViewValue ( view, requestIndex, isLittleEndian, type )

The abstract operation GetViewValue takes arguments view, requestIndex, isLittleEndian, and type. It is used by functions on DataView instances to retrieve values from the view's buffer. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(view, [[DataView]]).
  2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
  3. Let getIndex be ? ToIndex(requestIndex).
  4. Set isLittleEndian to ! ToBoolean(isLittleEndian).
  5. Let buffer be view.[[ViewedArrayBuffer]].
  6. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  7. Let viewOffset be view.[[ByteOffset]].
  8. Let viewSize be view.[[ByteLength]].
  9. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  10. If getIndex + elementSize > viewSize, throw aRangeErrorexception.
  11. Let bufferIndex be getIndex + viewOffset.
  12. ReturnGetValueFromBuffer(buffer, bufferIndex, type,false,Unordered, isLittleEndian).

25.3.1.2 SetViewValue ( view, requestIndex, isLittleEndian, type, value )

The abstract operation SetViewValue takes arguments view, requestIndex, isLittleEndian, type, and value. It is used by functions on DataView instances to store values into the view's buffer. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(view, [[DataView]]).
  2. Assert: view has a [[ViewedArrayBuffer]] internal slot.
  3. Let getIndex be ? ToIndex(requestIndex).
  4. If ! IsBigIntElementType(type) istrue, let numberValue be ? ToBigInt(value).
  5. Otherwise, let numberValue be ? ToNumber(value).
  6. Set isLittleEndian to ! ToBoolean(isLittleEndian).
  7. Let buffer be view.[[ViewedArrayBuffer]].
  8. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  9. Let viewOffset be view.[[ByteOffset]].
  10. Let viewSize be view.[[ByteLength]].
  11. Let elementSize be the Element Size value specified inTable 63for Element Type type.
  12. If getIndex + elementSize > viewSize, throw aRangeErrorexception.
  13. Let bufferIndex be getIndex + viewOffset.
  14. ReturnSetValueInBuffer(buffer, bufferIndex, type, numberValue,false,Unordered, isLittleEndian).

25.3.2 The DataView Constructor

The DataViewconstructor:

  • is %DataView%.
  • is the initial value of the"DataView"property of theglobal object.
  • creates and initializes a new DataView object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified DataView behaviour must include a super call to the DataViewconstructorto create and initialize subclass instances with the internal state necessary to support the DataView.prototype built-in methods.

25.3.2.1 DataView ( buffer [ , byteOffset [ , byteLength ] ] )

When the DataView function is called with at least one argument buffer, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Perform ? RequireInternalSlot(buffer, [[ArrayBufferData]]).
  3. Let offset be ? ToIndex(byteOffset).
  4. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  5. Let bufferByteLength be buffer.[[ArrayBufferByteLength]].
  6. If offset > bufferByteLength, throw aRangeErrorexception.
  7. If byteLength isundefined, then
    1. Let viewByteLength be bufferByteLength - offset.
  8. Else,
    1. Let viewByteLength be ? ToIndex(byteLength).
    2. If offset + viewByteLength > bufferByteLength, throw aRangeErrorexception.
  9. Let O be ? OrdinaryCreateFromConstructor(NewTarget,"%DataView.prototype%", « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »).
  10. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  11. Set O.[[ViewedArrayBuffer]] to buffer.
  12. Set O.[[ByteLength]] to viewByteLength.
  13. Set O.[[ByteOffset]] to offset.
  14. Return O.

25.3.3 Properties of the DataView Constructor

The DataViewconstructor:

25.3.3.1 DataView.prototype

The initial value of DataView.prototype is theDataView prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

25.3.4 Properties of the DataView Prototype Object

The DataView prototype object:

  • is %DataView.prototype%.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is anordinary object.
  • does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or [[ByteOffset]] internal slot.

25.3.4.1 get DataView.prototype.buffer

DataView.prototype.buffer is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. Return buffer.

25.3.4.2 get DataView.prototype.byteLength

DataView.prototype.byteLength is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  6. Let size be O.[[ByteLength]].
  7. Return𝔽(size).

25.3.4.3 get DataView.prototype.byteOffset

DataView.prototype.byteOffset is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[DataView]]).
  3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
  4. Let buffer be O.[[ViewedArrayBuffer]].
  5. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  6. Let offset be O.[[ByteOffset]].
  7. Return𝔽(offset).

25.3.4.4 DataView.prototype.constructor

The initial value of DataView.prototype.constructor is%DataView%.

25.3.4.5 DataView.prototype.getBigInt64 ( byteOffset [ , littleEndian ] )

When the getBigInt64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? GetViewValue(v, byteOffset, littleEndian,BigInt64).

25.3.4.6 DataView.prototype.getBigUint64 ( byteOffset [ , littleEndian ] )

When the getBigUint64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? GetViewValue(v, byteOffset, littleEndian,BigUint64).

25.3.4.7 DataView.prototype.getFloat32 ( byteOffset [ , littleEndian ] )

When the getFloat32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Float32).

25.3.4.8 DataView.prototype.getFloat64 ( byteOffset [ , littleEndian ] )

When the getFloat64 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Float64).

25.3.4.9 DataView.prototype.getInt8 ( byteOffset )

When the getInt8 method is called with argument byteOffset, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? GetViewValue(v, byteOffset,true,Int8).

25.3.4.10 DataView.prototype.getInt16 ( byteOffset [ , littleEndian ] )

When the getInt16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Int16).

25.3.4.11 DataView.prototype.getInt32 ( byteOffset [ , littleEndian ] )

When the getInt32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Int32).

25.3.4.12 DataView.prototype.getUint8 ( byteOffset )

When the getUint8 method is called with argument byteOffset, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? GetViewValue(v, byteOffset,true,Uint8).

25.3.4.13 DataView.prototype.getUint16 ( byteOffset [ , littleEndian ] )

When the getUint16 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Uint16).

25.3.4.14 DataView.prototype.getUint32 ( byteOffset [ , littleEndian ] )

When the getUint32 method is called with argument byteOffset and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? GetViewValue(v, byteOffset, littleEndian,Uint32).

25.3.4.15 DataView.prototype.setBigInt64 ( byteOffset, value [ , littleEndian ] )

When the setBigInt64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? SetViewValue(v, byteOffset, littleEndian,BigInt64, value).

25.3.4.16 DataView.prototype.setBigUint64 ( byteOffset, value [ , littleEndian ] )

When the setBigUint64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? SetViewValue(v, byteOffset, littleEndian,BigUint64, value).

25.3.4.17 DataView.prototype.setFloat32 ( byteOffset, value [ , littleEndian ] )

When the setFloat32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Float32, value).

25.3.4.18 DataView.prototype.setFloat64 ( byteOffset, value [ , littleEndian ] )

When the setFloat64 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Float64, value).

25.3.4.19 DataView.prototype.setInt8 ( byteOffset, value )

When the setInt8 method is called with arguments byteOffset and value, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? SetViewValue(v, byteOffset,true,Int8, value).

25.3.4.20 DataView.prototype.setInt16 ( byteOffset, value [ , littleEndian ] )

When the setInt16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Int16, value).

25.3.4.21 DataView.prototype.setInt32 ( byteOffset, value [ , littleEndian ] )

When the setInt32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Int32, value).

25.3.4.22 DataView.prototype.setUint8 ( byteOffset, value )

When the setUint8 method is called with arguments byteOffset and value, the following steps are taken:

  1. Let v be thethisvalue.
  2. Return ? SetViewValue(v, byteOffset,true,Uint8, value).

25.3.4.23 DataView.prototype.setUint16 ( byteOffset, value [ , littleEndian ] )

When the setUint16 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Uint16, value).

25.3.4.24 DataView.prototype.setUint32 ( byteOffset, value [ , littleEndian ] )

When the setUint32 method is called with arguments byteOffset and value and optional argument littleEndian, the following steps are taken:

  1. Let v be thethisvalue.
  2. If littleEndian is not present, set littleEndian tofalse.
  3. Return ? SetViewValue(v, byteOffset, littleEndian,Uint32, value).

25.3.4.25 DataView.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"DataView".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

25.3.5 Properties of DataView Instances

DataView instances are ordinary objects that inherit properties from theDataView prototype object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.

Note

The value of the [[DataView]] internal slot is not used within this specification. The simple presence of that internal slot is used within the specification to identify objects created using the DataViewconstructor.

25.4 The Atomics Object

The Atomics object:

  • is %Atomics%.
  • is the initial value of the"Atomics"property of theglobal object.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events. When used with discipline, the Atomics functions allow multi-agentprograms that communicate through shared memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory communication are provided by thememory model, defined below.

Note

For informative guidelines for programming and implementing shared memory in ECMAScript, please see the notes at the end of thememory modelsection.

25.4.1 WaiterList Objects

A WaiterList is a semantic object that contains an ordered list of those agents that are waiting on a location (block, i) in shared memory; block is aShared Data Blockand i a byte offset into the memory of block. A WaiterList object also optionally contains aSynchronize eventdenoting the previous leaving of its critical section.

Initially a WaiterList object has an empty list and noSynchronize event.

Theagent clusterhas a store of WaiterList objects; the store is indexed by (block, i). WaiterLists areagent-independent: a lookup in the store of WaiterLists by (block, i) will result in the same WaiterList object in anyagentin theagent cluster.

Each WaiterList has a critical section that controls exclusive access to that WaiterList during evaluation. Only a singleagentmay enter a WaiterList's critical section at one time. Entering and leaving a WaiterList's critical section is controlled by theabstract operationsEnterCriticalSectionandLeaveCriticalSection. Operations on a WaiterList—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting and retrieving theSynchronize event—may only be performed by agents that have entered the WaiterList's critical section.

25.4.2 Abstract Operations for Atomics

25.4.2.1 ValidateIntegerTypedArray ( typedArray [ , waitable ] )

The abstract operation ValidateIntegerTypedArray takes argument typedArray and optional argument waitable (a Boolean). It performs the following steps when called:

  1. If waitable is not present, set waitable tofalse.
  2. Perform ? ValidateTypedArray(typedArray).
  3. Let buffer be typedArray.[[ViewedArrayBuffer]].
  4. Let typeName be typedArray.[[TypedArrayName]].
  5. Let type be the Element Type value inTable 63for typeName.
  6. If waitable istrue, then
    1. If typeName is not"Int32Array"or"BigInt64Array", throw aTypeErrorexception.
  7. Else,
    1. If ! IsUnclampedIntegerElementType(type) isfalseand ! IsBigIntElementType(type) isfalse, throw aTypeErrorexception.
  8. Return buffer.

25.4.2.2 ValidateAtomicAccess ( typedArray, requestIndex )

The abstract operation ValidateAtomicAccess takes arguments typedArray and requestIndex. It performs the following steps when called:

  1. Assert: typedArray is an Object that has a [[ViewedArrayBuffer]] internal slot.
  2. Let length be typedArray.[[ArrayLength]].
  3. Let accessIndex be ? ToIndex(requestIndex).
  4. Assert: accessIndex ≥ 0.
  5. If accessIndexlength, throw aRangeErrorexception.
  6. Let arrayTypeName be typedArray.[[TypedArrayName]].
  7. Let elementSize be the Element Size value specified inTable 63for arrayTypeName.
  8. Let offset be typedArray.[[ByteOffset]].
  9. Return (accessIndex × elementSize) + offset.

25.4.2.3 GetWaiterList ( block, i )

The abstract operation GetWaiterList takes arguments block (aShared Data Block) and i (a non-negativeinteger). It performs the following steps when called:

  1. Assert: block is aShared Data Block.
  2. Assert: i and i + 3 are valid byte offsets within the memory of block.
  3. Assert: i is divisible by 4.
  4. Return theWaiterListthat is referenced by the pair (block, i).

25.4.2.4 EnterCriticalSection ( WL )

The abstract operation EnterCriticalSection takes argument WL (aWaiterList). It performs the following steps when called:

  1. Assert: The callingagentis not in thecritical sectionfor anyWaiterList.
  2. Wait until noagentis in thecritical sectionfor WL, then enter thecritical sectionfor WL (without allowing any otheragentto enter).
  3. If WL has aSynchronize event, then
    1. NOTE: A WL whosecritical sectionhas been entered at least once has aSynchronize eventset byLeaveCriticalSection.
    2. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
    3. Let eventsRecord be theAgent Events Recordin execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
    4. Let entererEventList be eventsRecord.[[EventList]].
    5. Let enterEvent be a newSynchronize event.
    6. Append enterEvent to entererEventList.
    7. Let leaveEvent be theSynchronize eventin WL.
    8. Append (leaveEvent, enterEvent) to eventsRecord.[[AgentSynchronizesWith]].

EnterCriticalSection has contention when anagentattempting to enter thecritical sectionmust wait for anotheragentto leave it. When there is no contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an implementation may choose an arbitrary order but may not cause anagentto wait indefinitely.

25.4.2.5 LeaveCriticalSection ( WL )

The abstract operation LeaveCriticalSection takes argument WL (aWaiterList). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Let execution be the [[CandidateExecution]] field of the calling surrounding'sAgent Record.
  3. Let eventsRecord be theAgent Events Recordin execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
  4. Let leaverEventList be eventsRecord.[[EventList]].
  5. Let leaveEvent be a newSynchronize event.
  6. Append leaveEvent to leaverEventList.
  7. Set theSynchronize eventin WL to leaveEvent.
  8. Leave thecritical sectionfor WL.

25.4.2.6 AddWaiter ( WL, W )

The abstract operation AddWaiter takes arguments WL (aWaiterList) and W (anagentsignifier). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Assert: W is not on the list of waiters in anyWaiterList.
  3. Add W to the end of the list of waiters in WL.

25.4.2.7 RemoveWaiter ( WL, W )

The abstract operation RemoveWaiter takes arguments WL (aWaiterList) and W (anagentsignifier). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Assert: W is on the list of waiters in WL.
  3. Remove W from the list of waiters in WL.

25.4.2.8 RemoveWaiters ( WL, c )

The abstract operation RemoveWaiters takes arguments WL (aWaiterList) and c (a non-negativeintegeror +∞). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Let L be a new emptyList.
  3. Let S be a reference to the list of waiters in WL.
  4. Repeat, while c > 0 and S is not an emptyList,
    1. Let W be the first waiter in S.
    2. Add W to the end of L.
    3. Remove W from S.
    4. If c is finite, set c to c - 1.
  5. Return L.

25.4.2.9 SuspendAgent ( WL, W, timeout )

The abstract operation SuspendAgent takes arguments WL (aWaiterList), W (anagentsignifier), and timeout (a non-negativeinteger). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Assert: W is equivalent toAgentSignifier().
  3. Assert: W is on the list of waiters in WL.
  4. Assert:AgentCanSuspend() istrue.
  5. PerformLeaveCriticalSection(WL) and suspend W for up to timeout milliseconds, performing the combined operation in such a way that a notification that arrives after thecritical sectionis exited but before the suspension takes effect is not lost. W can notify either because the timeout expired or because it was notified explicitly by anotheragentcallingNotifyWaiter(WL, W), and not for any other reasons at all.
  6. PerformEnterCriticalSection(WL).
  7. If W was notified explicitly by anotheragentcallingNotifyWaiter(WL, W), returntrue.
  8. Returnfalse.

25.4.2.10 NotifyWaiter ( WL, W )

The abstract operation NotifyWaiter takes arguments WL (aWaiterList) and W (anagentsignifier). It performs the following steps when called:

  1. Assert: The callingagentis in thecritical sectionfor WL.
  2. Notify theagentW.
Note

The embedding may delay notifying W, e.g. for resource management reasons, but W must eventually be notified in order to guarantee forward progress.

25.4.2.11 AtomicReadModifyWrite ( typedArray, index, value, op )

The abstract operation AtomicReadModifyWrite takes arguments typedArray, index, value, and op (aread-modify-write modification function). op takes twoListof byte values arguments and returns aListof byte values. This operation atomically loads a value, combines it with another value, and stores the result of the combination. It returns the loaded value. It performs the following steps when called:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. Let arrayTypeName be typedArray.[[TypedArrayName]].
  4. If typedArray.[[ContentType]] isBigInt, let v be ? ToBigInt(value).
  5. Otherwise, let v be𝔽(?ToIntegerOrInfinity(value)).
  6. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  7. NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  8. Let elementType be the Element Type value inTable 63for arrayTypeName.
  9. ReturnGetModifySetValueInBuffer(buffer, indexedPosition, elementType, v, op).

25.4.2.12 ByteListBitwiseOp ( op, xBytes, yBytes )

The abstract operation ByteListBitwiseOp takes arguments op (a sequence of Unicode code points), xBytes (aListof byte values), and yBytes (aListof byte values). The operation atomically performs a bitwise operation on all byte values of the arguments and returns aListof byte values. It performs the following steps when called:

  1. Assert: op is &, ^, or |.
  2. Assert: xBytes and yBytes have the same number of elements.
  3. Let result be a new emptyList.
  4. Let i be 0.
  5. For each element xByte of xBytes, do
    1. Let yByte be yBytes[i].
    2. If op is &, let resultByte be the result of applying the bitwise AND operation to xByte and yByte.
    3. Else if op is ^, let resultByte be the result of applying the bitwise exclusive OR (XOR) operation to xByte and yByte.
    4. Else, op is |. Let resultByte be the result of applying the bitwise inclusive OR operation to xByte and yByte.
    5. Set i to i + 1.
    6. Append resultByte to the end of result.
  6. Return result.

25.4.2.13 ByteListEqual ( xBytes, yBytes )

The abstract operation ByteListEqual takes arguments xBytes (aListof byte values) and yBytes (aListof byte values). It performs the following steps when called:

  1. If xBytes and yBytes do not have the same number of elements, returnfalse.
  2. Let i be 0.
  3. For each element xByte of xBytes, do
    1. Let yByte be yBytes[i].
    2. If xByteyByte, returnfalse.
    3. Set i to i + 1.
  4. Returntrue.

25.4.3 Atomics.add ( typedArray, index, value )

The following steps are taken:

  1. Let type be the Element Type value inTable 63for typedArray.[[TypedArrayName]].
  2. Let isLittleEndian be the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  3. Let add be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
    1. Let x beRawBytesToNumeric(type, xBytes, isLittleEndian).
    2. Let y beRawBytesToNumeric(type, yBytes, isLittleEndian).
    3. Let T beType(x).
    4. Let sum be T::add(x, y).
    5. Let sumBytes beNumericToRawBytes(type, sum, isLittleEndian).
    6. Assert: sumBytes, xBytes, and yBytes have the same number of elements.
    7. Return sumBytes.
  4. Return ? AtomicReadModifyWrite(typedArray, index, value, add).

25.4.4 Atomics.and ( typedArray, index, value )

The following steps are taken:

  1. Let and be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. ReturnByteListBitwiseOp(&, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, and).

25.4.5 Atomics.compareExchange ( typedArray, index, expectedValue, replacementValue )

The following steps are taken:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let block be buffer.[[ArrayBufferData]].
  3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  4. Let arrayTypeName be typedArray.[[TypedArrayName]].
  5. If typedArray.[[ContentType]] isBigInt, then
    1. Let expected be ? ToBigInt(expectedValue).
    2. Let replacement be ? ToBigInt(replacementValue).
  6. Else,
    1. Let expected be𝔽(?ToIntegerOrInfinity(expectedValue)).
    2. Let replacement be𝔽(?ToIntegerOrInfinity(replacementValue)).
  7. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  8. NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  9. Let elementType be the Element Type value inTable 63for arrayTypeName.
  10. Let elementSize be the Element Size value specified inTable 63for Element Type elementType.
  11. Let isLittleEndian be the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  12. Let expectedBytes beNumericToRawBytes(elementType, expected, isLittleEndian).
  13. Let replacementBytes beNumericToRawBytes(elementType, replacement, isLittleEndian).
  14. IfIsSharedArrayBuffer(buffer) istrue, then
    1. Let execution be the [[CandidateExecution]] field of thesurrounding agent'sAgent Record.
    2. Let eventList be the [[EventList]] field of the element in execution.[[EventsRecords]] whose [[AgentSignifier]] isAgentSignifier().
    3. Let rawBytesRead be aListof length elementSize whose elements are nondeterministically chosen byte values.
    4. NOTE: In implementations, rawBytesRead is the result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the underlying hardware. The nondeterminism is a semantic prescription of thememory modelto describe observable behaviour of hardware with weak consistency.
    5. NOTE: The comparison of the expected value and the read value is performed outside of theread-modify-write modification functionto avoid needlessly strong synchronization when the expected value is not equal to the read value.
    6. IfByteListEqual(rawBytesRead, expectedBytes) istrue, then
      1. Let second be a newread-modify-write modification functionwith parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
        1. Return newBytes.
      2. Let event beReadModifyWriteSharedMemory{ [[Order]]:SeqCst, [[NoTear]]:true, [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
    7. Else,
      1. Let event beReadSharedMemory{ [[Order]]:SeqCst, [[NoTear]]:true, [[Block]]: block, [[ByteIndex]]: indexedPosition, [[ElementSize]]: elementSize }.
    8. Append event to eventList.
    9. AppendChosen Value Record{ [[Event]]: event, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
  15. Else,
    1. Let rawBytesRead be aListof length elementSize whose elements are the sequence of elementSize bytes starting with block[indexedPosition].
    2. IfByteListEqual(rawBytesRead, expectedBytes) istrue, then
      1. Store the individual bytes of replacementBytes into block, starting at block[indexedPosition].
  16. ReturnRawBytesToNumeric(elementType, rawBytesRead, isLittleEndian).

25.4.6 Atomics.exchange ( typedArray, index, value )

The following steps are taken:

  1. Let second be a newread-modify-write modification functionwith parameters (oldBytes, newBytes) that captures nothing and performs the following steps atomically when called:
    1. Return newBytes.
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, second).

25.4.7 Atomics.isLockFree ( size )

The following steps are taken:

  1. Let n be ? ToIntegerOrInfinity(size).
  2. Let AR be theAgent Recordof thesurrounding agent.
  3. If n = 1, return AR.[[IsLockFree1]].
  4. If n = 2, return AR.[[IsLockFree2]].
  5. If n = 4, returntrue.
  6. If n = 8, return AR.[[IsLockFree8]].
  7. Returnfalse.
Note

Atomics.isLockFree() is an optimization primitive. The intuition is that if the atomic step of an atomic primitive (compareExchange, load, store, add, sub, and, or, xor, or exchange) on a datum of size n bytes will be performed without the callingagentacquiring a lock outside the n bytes comprising the datum, then Atomics.isLockFree(n) will returntrue. High-performance algorithms will use Atomics.isLockFree to determine whether to use locks or atomic operations in critical sections. If an atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own locking.

Atomics.isLockFree(4) always returnstrueas that can be supported on all known relevant hardware. Being able to assume this will generally simplify programs.

Regardless of the value of Atomics.isLockFree, all atomic operations are guaranteed to be atomic. For example, they will never have a visible operation take place in the middle of the operation (e.g., "tearing").

25.4.8 Atomics.load ( typedArray, index )

The following steps are taken:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  4. NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toValidateAtomicAccesson the preceding line can have arbitrary side effects, which could cause the buffer to become detached.
  5. Let arrayTypeName be typedArray.[[TypedArrayName]].
  6. Let elementType be the Element Type value inTable 63for arrayTypeName.
  7. ReturnGetValueFromBuffer(buffer, indexedPosition, elementType,true,SeqCst).

25.4.9 Atomics.or ( typedArray, index, value )

The following steps are taken:

  1. Let or be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. ReturnByteListBitwiseOp(|, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, or).

25.4.10 Atomics.store ( typedArray, index, value )

The following steps are taken:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. Let arrayTypeName be typedArray.[[TypedArrayName]].
  4. If arrayTypeName is"BigUint64Array"or"BigInt64Array", let v be ? ToBigInt(value).
  5. Otherwise, let v be𝔽(?ToIntegerOrInfinity(value)).
  6. IfIsDetachedBuffer(buffer) istrue, throw aTypeErrorexception.
  7. NOTE: The above check is not redundant with the check inValidateIntegerTypedArraybecause the call toToBigIntorToIntegerOrInfinityon the preceding lines can have arbitrary side effects, which could cause the buffer to become detached.
  8. Let elementType be the Element Type value inTable 63for arrayTypeName.
  9. PerformSetValueInBuffer(buffer, indexedPosition, elementType, v,true,SeqCst).
  10. Return v.

25.4.11 Atomics.sub ( typedArray, index, value )

The following steps are taken:

  1. Let type be the Element Type value inTable 63for typedArray.[[TypedArrayName]].
  2. Let isLittleEndian be the value of the [[LittleEndian]] field of thesurrounding agent'sAgent Record.
  3. Let subtract be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures type and isLittleEndian and performs the following steps atomically when called:
    1. Let x beRawBytesToNumeric(type, xBytes, isLittleEndian).
    2. Let y beRawBytesToNumeric(type, yBytes, isLittleEndian).
    3. Let T beType(x).
    4. Let difference be T::subtract(x, y).
    5. Let differenceBytes beNumericToRawBytes(type, difference, isLittleEndian).
    6. Assert: differenceBytes, xBytes, and yBytes have the same number of elements.
    7. Return differenceBytes.
  4. Return ? AtomicReadModifyWrite(typedArray, index, value, subtract).

25.4.12 Atomics.wait ( typedArray, index, value, timeout )

Atomics.wait puts the callingagentin a wait queue and puts it to sleep until it is notified or the sleep times out. The following steps are taken:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray,true).
  2. IfIsSharedArrayBuffer(buffer) isfalse, throw aTypeErrorexception.
  3. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  4. Let arrayTypeName be typedArray.[[TypedArrayName]].
  5. If arrayTypeName is"BigInt64Array", let v be ? ToBigInt64(value).
  6. Otherwise, let v be ? ToInt32(value).
  7. Let q be ? ToNumber(timeout).
  8. If q isNaNor+∞𝔽, let t be +∞; else if q is-∞𝔽, let t be 0; else let t bemax((q), 0).
  9. Let B beAgentCanSuspend().
  10. If B isfalse, throw aTypeErrorexception.
  11. Let block be buffer.[[ArrayBufferData]].
  12. Let WL beGetWaiterList(block, indexedPosition).
  13. PerformEnterCriticalSection(WL).
  14. Let elementType be the Element Type value inTable 63for arrayTypeName.
  15. Let w be ! GetValueFromBuffer(buffer, indexedPosition, elementType,true,SeqCst).
  16. If vw, then
    1. PerformLeaveCriticalSection(WL).
    2. Return the String"not-equal".
  17. Let W beAgentSignifier().
  18. PerformAddWaiter(WL, W).
  19. Let notified beSuspendAgent(WL, W, t).
  20. If notified istrue, then
    1. Assert: W is not on the list of waiters in WL.
  21. Else,
    1. PerformRemoveWaiter(WL, W).
  22. PerformLeaveCriticalSection(WL).
  23. If notified istrue, return the String"ok".
  24. Return the String"timed-out".

25.4.13 Atomics.notify ( typedArray, index, count )

Atomics.notify notifies some agents that are sleeping in the wait queue. The following steps are taken:

  1. Let buffer be ? ValidateIntegerTypedArray(typedArray,true).
  2. Let indexedPosition be ? ValidateAtomicAccess(typedArray, index).
  3. If count isundefined, let c be +∞.
  4. Else,
    1. Let intCount be ? ToIntegerOrInfinity(count).
    2. Let c bemax(intCount, 0).
  5. Let block be buffer.[[ArrayBufferData]].
  6. Let arrayTypeName be typedArray.[[TypedArrayName]].
  7. IfIsSharedArrayBuffer(buffer) isfalse, return+0𝔽.
  8. Let WL beGetWaiterList(block, indexedPosition).
  9. Let n be 0.
  10. PerformEnterCriticalSection(WL).
  11. Let S beRemoveWaiters(WL, c).
  12. Repeat, while S is not an emptyList,
    1. Let W be the firstagentin S.
    2. Remove W from the front of S.
    3. PerformNotifyWaiter(WL, W).
    4. Set n to n + 1.
  13. PerformLeaveCriticalSection(WL).
  14. Return𝔽(n).

25.4.14 Atomics.xor ( typedArray, index, value )

The following steps are taken:

  1. Let xor be a newread-modify-write modification functionwith parameters (xBytes, yBytes) that captures nothing and performs the following steps atomically when called:
    1. ReturnByteListBitwiseOp(^, xBytes, yBytes).
  2. Return ? AtomicReadModifyWrite(typedArray, index, value, xor).

25.4.15 Atomics [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Atomics".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

25.5 The JSON Object

The JSON object:

  • is %JSON%.
  • is the initial value of the"JSON"property of theglobal object.
  • is anordinary object.
  • contains two functions, parse and stringify, that are used to parse and construct JSON texts.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange format described in the ECMA-404 specification without any deletions or extensions to the format.

25.5.1 JSON.parse ( text [ , reviver ] )

The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The JSON format represents literals, arrays, and objects with a syntax similar to the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, andnull.

The optional reviver parameter is a function that takes two parameters, key and value. It can filter and transform the results. It is called with each of the key/value pairs produced by the parse, and its return value is used instead of the original value. If it returns what it received, the structure is not modified. If it returnsundefinedthen the property is deleted from the result.

  1. Let jsonString be ? ToString(text).
  2. Parse ! StringToCodePoints(jsonString) as a JSON text as specified in ECMA-404. Throw aSyntaxErrorexception if it is not a valid JSON text as defined in that specification.
  3. Let scriptString be thestring-concatenationof"(", jsonString, and");".
  4. Let script beParseText(!StringToCodePoints(scriptString),Script).
  5. NOTE: Theearly errorrules defined in13.2.5.1have special handling for the above invocation ofParseText.
  6. Assert: script is aParse Node.
  7. Let completion be the result of evaluating script.
  8. NOTE: ThePropertyDefinitionEvaluationsemantics defined in13.2.5.5have special handling for the above evaluation.
  9. Let unfiltered be completion.[[Value]].
  10. Assert: unfiltered is either a String, Number, Boolean, Null, or an Object that is defined by either anArrayLiteralor anObjectLiteral.
  11. IfIsCallable(reviver) istrue, then
    1. Let root be ! OrdinaryObjectCreate(%Object.prototype%).
    2. Let rootName be the empty String.
    3. Perform ! CreateDataPropertyOrThrow(root, rootName, unfiltered).
    4. Return ? InternalizeJSONProperty(root, rootName, reviver).
  12. Else,
    1. Return unfiltered.

The"length"property of the parse function is2𝔽.

Note

Valid JSON text is a subset of the ECMAScriptPrimaryExpressionsyntax. Step2verifies that jsonString conforms to that subset, and step10asserts that that parsing and evaluation returns a value of an appropriate type.

However, because13.2.5.5behaves differently during JSON.parse, the same source text can produce different results when evaluated as aPrimaryExpressionrather than as JSON. Furthermore, the Early Error for duplicate"__proto__"properties in object literals, which likewise does not apply during JSON.parse, means that not all texts accepted by JSON.parse are valid as aPrimaryExpression, despite matching the grammar.

25.5.1.1 InternalizeJSONProperty ( holder, name, reviver )

The abstract operation InternalizeJSONProperty takes arguments holder (an Object), name (a String), and reviver (afunction object).

Note 1

This algorithm intentionally does not throw an exception if either [[Delete]] orCreateDataPropertyreturnfalse.

It performs the following steps when called:

  1. Let val be ? Get(holder, name).
  2. IfType(val) is Object, then
    1. Let isArray be ? IsArray(val).
    2. If isArray istrue, then
      1. Let I be 0.
      2. Let len be ? LengthOfArrayLike(val).
      3. Repeat, while I < len,
        1. Let prop be ! ToString(𝔽(I)).
        2. Let newElement be ? InternalizeJSONProperty(val, prop, reviver).
        3. If newElement isundefined, then
          1. Perform ? val.[[Delete]](prop).
        4. Else,
          1. Perform ? CreateDataProperty(val, prop, newElement).
        5. Set I to I + 1.
    3. Else,
      1. Let keys be ? EnumerableOwnPropertyNames(val,key).
      2. For each String P of keys, do
        1. Let newElement be ? InternalizeJSONProperty(val, P, reviver).
        2. If newElement isundefined, then
          1. Perform ? val.[[Delete]](P).
        3. Else,
          1. Perform ? CreateDataProperty(val, P, newElement).
  3. Return ? Call(reviver, holder, « name, val »).

It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do so by defining a different parse function.

Note 2

In the case where there are duplicate name Strings within an object, lexically preceding values for the same key shall be overwritten.

25.5.2 JSON.stringify ( value [ , replacer [ , space ] ] )

The stringify function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, orundefined. It can take three parameters. The value parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number ornull. The optional replacer parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional space parameter is a String or Number that allows the result to have white space injected into it to improve human readability.

These are the steps in stringifying an object:

  1. Let stack be a new emptyList.
  2. Let indent be the empty String.
  3. Let PropertyList and ReplacerFunction beundefined.
  4. IfType(replacer) is Object, then
    1. IfIsCallable(replacer) istrue, then
      1. Set ReplacerFunction to replacer.
    2. Else,
      1. Let isArray be ? IsArray(replacer).
      2. If isArray istrue, then
        1. Set PropertyList to a new emptyList.
        2. Let len be ? LengthOfArrayLike(replacer).
        3. Let k be 0.
        4. Repeat, while k < len,
          1. Let prop be ! ToString(𝔽(k)).
          2. Let v be ? Get(replacer, prop).
          3. Let item beundefined.
          4. IfType(v) is String, set item to v.
          5. Else ifType(v) is Number, set item to ! ToString(v).
          6. Else ifType(v) is Object, then
            1. If v has a [[StringData]] or [[NumberData]] internal slot, set item to ? ToString(v).
          7. If item is notundefinedand item is not currently an element of PropertyList, then
            1. Append item to the end of PropertyList.
          8. Set k to k + 1.
  5. IfType(space) is Object, then
    1. If space has a [[NumberData]] internal slot, then
      1. Set space to ? ToNumber(space).
    2. Else if space has a [[StringData]] internal slot, then
      1. Set space to ? ToString(space).
  6. IfType(space) is Number, then
    1. Let spaceMV be ! ToIntegerOrInfinity(space).
    2. Set spaceMV tomin(10, spaceMV).
    3. If spaceMV < 1, let gap be the empty String; otherwise let gap be the String value containing spaceMV occurrences of the code unit 0x0020 (SPACE).
  7. Else ifType(space) is String, then
    1. If the length of space is 10 or less, let gap be space; otherwise let gap be thesubstringof space from 0 to 10.
  8. Else,
    1. Let gap be the empty String.
  9. Let wrapper be ! OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(wrapper, the empty String, value).
  11. Let state be theRecord{ [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
  12. Return ? SerializeJSONProperty(state, the empty String, wrapper).

The"length"property of the stringify function is3𝔽.

Note 1

JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic structure, then the stringify function must throw aTypeErrorexception. This is an example of a value that cannot be stringified:

a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2

Symbolic primitive values are rendered as follows:

  • Thenullvalue is rendered in JSON text as the String"null".
  • Theundefinedvalue is not rendered.
  • Thetruevalue is rendered in JSON text as the String"true".
  • Thefalsevalue is rendered in JSON text as the String"false".
Note 3

String values are wrapped in QUOTATION MARK (") code units. The code units " and \ are escaped with \ prefixes. Control characters code units are replaced with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE), \f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN), \t (CHARACTER TABULATION).

Note 4

Finite numbers are stringified as if by callingToString(number).NaNandInfinityregardless of sign are represented as the String"null".

Note 5

Values that do not have a JSON representation (such asundefinedand functions) do not produce a String. Instead they produce theundefinedvalue. In arrays these values are represented as the String"null". In objects an unrepresentable value causes the property to be excluded from stringification.

Note 6

An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String representing the key orproperty name, a U+003A (COLON), and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE BRACKET).

25.5.2.1 SerializeJSONProperty ( state, key, holder )

The abstract operation SerializeJSONProperty takes arguments state, key, and holder. It performs the following steps when called:

  1. Let value be ? Get(holder, key).
  2. IfType(value) is Object or BigInt, then
    1. Let toJSON be ? GetV(value,"toJSON").
    2. IfIsCallable(toJSON) istrue, then
      1. Set value to ? Call(toJSON, value, « key »).
  3. If state.[[ReplacerFunction]] is notundefined, then
    1. Set value to ? Call(state.[[ReplacerFunction]], holder, « key, value »).
  4. IfType(value) is Object, then
    1. If value has a [[NumberData]] internal slot, then
      1. Set value to ? ToNumber(value).
    2. Else if value has a [[StringData]] internal slot, then
      1. Set value to ? ToString(value).
    3. Else if value has a [[BooleanData]] internal slot, then
      1. Set value to value.[[BooleanData]].
    4. Else if value has a [[BigIntData]] internal slot, then
      1. Set value to value.[[BigIntData]].
  5. If value isnull, return"null".
  6. If value istrue, return"true".
  7. If value isfalse, return"false".
  8. IfType(value) is String, returnQuoteJSONString(value).
  9. IfType(value) is Number, then
    1. If value is finite, return ! ToString(value).
    2. Return"null".
  10. IfType(value) is BigInt, throw aTypeErrorexception.
  11. IfType(value) is Object andIsCallable(value) isfalse, then
    1. Let isArray be ? IsArray(value).
    2. If isArray istrue, return ? SerializeJSONArray(state, value).
    3. Return ? SerializeJSONObject(state, value).
  12. Returnundefined.

25.5.2.2 QuoteJSONString ( value )

The abstract operation QuoteJSONString takes argument value. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units within it. This operation interprets value as a sequence of UTF-16 encoded code points, as described in6.1.4. It performs the following steps when called:

  1. Let product be the String value consisting solely of the code unit 0x0022 (QUOTATION MARK).
  2. For each code point C of ! StringToCodePoints(value), do
    1. If C is listed in the “Code Point” column ofTable 64, then
      1. Set product to thestring-concatenationof product and the escape sequence for C as specified in the “Escape Sequence” column of the corresponding row.
    2. Else if C has a numeric value less than 0x0020 (SPACE), or if C has the same numeric value as aleading surrogateortrailing surrogate, then
      1. Let unit be the code unit whose numeric value is that of C.
      2. Set product to thestring-concatenationof product andUnicodeEscape(unit).
    3. Else,
      1. Set product to thestring-concatenationof product and ! UTF16EncodeCodePoint(C).
  3. Set product to thestring-concatenationof product and the code unit 0x0022 (QUOTATION MARK).
  4. Return product.
Table 64: JSON Single Character Escape Sequences
Code PointUnicode Character NameEscape Sequence
U+0008BACKSPACE\b
U+0009CHARACTER TABULATION\t
U+000ALINE FEED (LF)\n
U+000CFORM FEED (FF)\f
U+000DCARRIAGE RETURN (CR)\r
U+0022QUOTATION MARK\"
U+005CREVERSE SOLIDUS\\

25.5.2.3 UnicodeEscape ( C )

The abstract operation UnicodeEscape takes argument C (a code unit). It represents C as a Unicode escape sequence. It performs the following steps when called:

  1. Let n be the numeric value of C.
  2. Assert: n ≤ 0xFFFF.
  3. Return thestring-concatenationof:
    • the code unit 0x005C (REVERSE SOLIDUS)
    • "u"
    • the String representation of n, formatted as a four-digit lowercase hexadecimal number, padded to the left with zeroes if necessary

25.5.2.4 SerializeJSONObject ( state, value )

The abstract operation SerializeJSONObject takes arguments state and value. It serializes an object. It performs the following steps when called:

  1. If state.[[Stack]] contains value, throw aTypeErrorexception because the structure is cyclical.
  2. Append value to state.[[Stack]].
  3. Let stepback be state.[[Indent]].
  4. Set state.[[Indent]] to thestring-concatenationof state.[[Indent]] and state.[[Gap]].
  5. If state.[[PropertyList]] is notundefined, then
    1. Let K be state.[[PropertyList]].
  6. Else,
    1. Let K be ? EnumerableOwnPropertyNames(value,key).
  7. Let partial be a new emptyList.
  8. For each element P of K, do
    1. Let strP be ? SerializeJSONProperty(state, P, value).
    2. If strP is notundefined, then
      1. Let member beQuoteJSONString(P).
      2. Set member to thestring-concatenationof member and":".
      3. If state.[[Gap]] is not the empty String, then
        1. Set member to thestring-concatenationof member and the code unit 0x0020 (SPACE).
      4. Set member to thestring-concatenationof member and strP.
      5. Append member to partial.
  9. If partial is empty, then
    1. Let final be"{}".
  10. Else,
    1. If state.[[Gap]] is the empty String, then
      1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
      2. Let final be thestring-concatenationof"{", properties, and"}".
    2. Else,
      1. Let separator be thestring-concatenationof the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
      2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
      3. Let final be thestring-concatenationof"{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"}".
  11. Remove the last element of state.[[Stack]].
  12. Set state.[[Indent]] to stepback.
  13. Return final.

25.5.2.5 SerializeJSONArray ( state, value )

The abstract operation SerializeJSONArray takes arguments state and value. It serializes an array. It performs the following steps when called:

  1. If state.[[Stack]] contains value, throw aTypeErrorexception because the structure is cyclical.
  2. Append value to state.[[Stack]].
  3. Let stepback be state.[[Indent]].
  4. Set state.[[Indent]] to thestring-concatenationof state.[[Indent]] and state.[[Gap]].
  5. Let partial be a new emptyList.
  6. Let len be ? LengthOfArrayLike(value).
  7. Let index be 0.
  8. Repeat, while index < len,
    1. Let strP be ? SerializeJSONProperty(state, ! ToString(𝔽(index)), value).
    2. If strP isundefined, then
      1. Append"null"to partial.
    3. Else,
      1. Append strP to partial.
    4. Set index to index + 1.
  9. If partial is empty, then
    1. Let final be"[]".
  10. Else,
    1. If state.[[Gap]] is the empty String, then
      1. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first String or after the last String.
      2. Let final be thestring-concatenationof"[", properties, and"]".
    2. Else,
      1. Let separator be thestring-concatenationof the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
      2. Let properties be the String value formed by concatenating all the element Strings of partial with each adjacent pair of Strings separated with separator. The separator String is not inserted either before the first String or after the last String.
      3. Let final be thestring-concatenationof"[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED), stepback, and"]".
  11. Remove the last element of state.[[Stack]].
  12. Set state.[[Indent]] to stepback.
  13. Return final.
Note

The representation of arrays includes only the elements between zero andarray.length - 1inclusive. Properties whose keys are notarray indexesare excluded from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.

25.5.3 JSON [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"JSON".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

26 Managing Memory

26.1 WeakRef Objects

AWeakRefis an object that is used to refer to a target object without preserving it from garbage collection. WeakRefs can be dereferenced to allow access to the target object, if the target object hasn't been reclaimed by garbage collection.

26.1.1 The WeakRef Constructor

The WeakRefconstructor:

  • is %WeakRef%.
  • is the initial value of the"WeakRef"property of theglobal object.
  • creates and initializes a new WeakRef object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified WeakRef behaviour must include a super call to the WeakRefconstructorto create and initialize the subclass instance with the internal state necessary to support the WeakRef.prototype built-in methods.

26.1.1.1 WeakRef ( target )

When the WeakRef function is called with argument target, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. IfType(target) is not Object, throw aTypeErrorexception.
  3. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget,"%WeakRef.prototype%", « [[WeakRefTarget]] »).
  4. Perform ! AddToKeptObjects(target).
  5. Set weakRef.[[WeakRefTarget]] to target.
  6. Return weakRef.

26.1.2 Properties of the WeakRef Constructor

TheWeakRefconstructor:

26.1.2.1 WeakRef.prototype

The initial value of WeakRef.prototype is theWeakRef prototypeobject.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

26.1.3 Properties of the WeakRef Prototype Object

The WeakRef prototype object:

26.1.3.1 WeakRef.prototype.constructor

The initial value of WeakRef.prototype.constructor is%WeakRef%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

26.1.3.2 WeakRef.prototype.deref ( )

The following steps are taken:

  1. Let weakRef be thethisvalue.
  2. Perform ? RequireInternalSlot(weakRef, [[WeakRefTarget]]).
  3. Return ! WeakRefDeref(weakRef).
Note

If theWeakRefreturns a target Object that is notundefined, then this target object should not be garbage collected until the current execution of ECMAScript code has completed. TheAddToKeptObjectsoperation makes sure read consistency is maintained.

target = { foo: function() {} };
let weakRef = new WeakRef(target);

... later ...

if (weakRef.deref()) {
  weakRef.deref().foo();
}

In the above example, if the first deref does not evaluate toundefinedthen the second deref cannot either.

26.1.3.3 WeakRef.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"WeakRef".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

26.1.4 WeakRef Abstract Operations

26.1.4.1 WeakRefDeref ( weakRef )

The abstract operation WeakRefDeref takes argument weakRef (aWeakRef). It performs the following steps when called:

  1. Let target be weakRef.[[WeakRefTarget]].
  2. If target is notempty, then
    1. Perform ! AddToKeptObjects(target).
    2. Return target.
  3. Returnundefined.
Note

This abstract operation is defined separately from WeakRef.prototype.deref strictly to make it possible to succinctly define liveness.

26.1.5 Properties of WeakRef Instances

WeakRefinstances are ordinary objects that inherit properties from theWeakRef prototype.WeakRefinstances also have a [[WeakRefTarget]] internal slot.

26.2 FinalizationRegistry Objects

AFinalizationRegistryis an object that manages registration and unregistration of cleanup operations that are performed when target objects are garbage collected.

26.2.1 The FinalizationRegistry Constructor

The FinalizationRegistryconstructor:

  • is %FinalizationRegistry%.
  • is the initial value of the"FinalizationRegistry"property of theglobal object.
  • creates and initializes a new FinalizationRegistry object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified FinalizationRegistry behaviour must include a super call to the FinalizationRegistryconstructorto create and initialize the subclass instance with the internal state necessary to support the FinalizationRegistry.prototype built-in methods.

26.2.1.1 FinalizationRegistry ( cleanupCallback )

When the FinalizationRegistry function is called with argument cleanupCallback, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. IfIsCallable(cleanupCallback) isfalse, throw aTypeErrorexception.
  3. Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
  4. Let fn be theactive function object.
  5. Set finalizationRegistry.[[Realm]] to fn.[[Realm]].
  6. Set finalizationRegistry.[[CleanupCallback]] toHostMakeJobCallback(cleanupCallback).
  7. Set finalizationRegistry.[[Cells]] to a new emptyList.
  8. Return finalizationRegistry.

26.2.2 Properties of the FinalizationRegistry Constructor

TheFinalizationRegistryconstructor:

26.2.2.1 FinalizationRegistry.prototype

The initial value of FinalizationRegistry.prototype is theFinalizationRegistry prototypeobject.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

26.2.3 Properties of the FinalizationRegistry Prototype Object

The FinalizationRegistry prototype object:

  • is %FinalizationRegistry.prototype%.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is anordinary object.
  • does not have [[Cells]] and [[CleanupCallback]] internal slots.

26.2.3.1 FinalizationRegistry.prototype.constructor

The initial value of FinalizationRegistry.prototype.constructor is%FinalizationRegistry%.

26.2.3.2 FinalizationRegistry.prototype.register ( target, heldValue [ , unregisterToken ] )

The following steps are taken:

  1. Let finalizationRegistry be thethisvalue.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. IfType(target) is not Object, throw aTypeErrorexception.
  4. IfSameValue(target, heldValue) istrue, throw aTypeErrorexception.
  5. IfType(unregisterToken) is not Object, then
    1. If unregisterToken is notundefined, throw aTypeErrorexception.
    2. Set unregisterToken toempty.
  6. Let cell be theRecord{ [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
  7. Append cell to finalizationRegistry.[[Cells]].
  8. Returnundefined.
Note

Based on the algorithms and definitions in this specification, cell.[[HeldValue]] islivewhen cell is in finalizationRegistry.[[Cells]]; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] arelive. For example, registering an object with itself as its unregister token would not keep the object alive forever.

26.2.3.3 FinalizationRegistry.prototype.unregister ( unregisterToken )

The following steps are taken:

  1. Let finalizationRegistry be thethisvalue.
  2. Perform ? RequireInternalSlot(finalizationRegistry, [[Cells]]).
  3. IfType(unregisterToken) is not Object, throw aTypeErrorexception.
  4. Let removed befalse.
  5. For eachRecord{ [[WeakRefTarget]], [[HeldValue]], [[UnregisterToken]] } cell of finalizationRegistry.[[Cells]], do
    1. If cell.[[UnregisterToken]] is notemptyandSameValue(cell.[[UnregisterToken]], unregisterToken) istrue, then
      1. Remove cell from finalizationRegistry.[[Cells]].
      2. Set removed totrue.
  6. Return removed.

26.2.3.4 FinalizationRegistry.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"FinalizationRegistry".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

26.2.4 Properties of FinalizationRegistry Instances

FinalizationRegistryinstances are ordinary objects that inherit properties from theFinalizationRegistry prototype.FinalizationRegistryinstances also have [[Cells]] and [[CleanupCallback]] internal slots.

27 Control Abstraction Objects

27.1 Iteration

27.1.1 Common Iteration Interfaces

An interface is a set of property keys whose associated values match a specific specification. Any object that provides all the properties as described by an interface's specification conforms to that interface. An interface is not represented by a distinct object. There may be many separately implemented objects that conform to any interface. An individual object may conform to multiple interfaces.

27.1.1.1 The Iterable Interface

The Iterable interface includes the property described inTable 65:

Table 65: Iterable Interface Required Properties
PropertyValueRequirements
@@iteratorA function that returns an Iterator object.The returned object must conform to the Iterator interface.

27.1.1.2 The Iterator Interface

An object that implements the Iterator interface must include the property inTable 66. Such objects may also implement the properties inTable 67.

Table 66: Iterator Interface Required Properties
PropertyValueRequirements
"next"A function that returns an IteratorResult object.The returned object must conform to the IteratorResult interface. If a previous call to the next method of an Iterator has returned an IteratorResult object whose"done"property istrue, then all subsequent calls to the next method of that object should also return an IteratorResult object whose"done"property istrue. However, this requirement is not enforced.
Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target Iterator. The for-of statement and other common users of Iterators do not pass any arguments, so Iterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.

Table 67: Iterator Interface Optional Properties
PropertyValueRequirements
"return"A function that returns an IteratorResult object.The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller does not intend to make any more next method calls to the Iterator. The returned IteratorResult object will typically have a"done"property whose value istrue, and a"value"property with the value passed as the argument of the return method. However, this requirement is not enforced.
"throw"A function that returns an IteratorResult object.The returned object must conform to the IteratorResult interface. Invoking this method notifies the Iterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to throw the value passed as the argument. If the method does not throw, the returned IteratorResult object will typically have a"done"property whose value istrue.
Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-of, yield*, and array destructuring call these methods after performing an existence check. Most ECMAScript library functions that accept Iterable objects as arguments also conditionally call them.

27.1.1.3 The AsyncIterable Interface

The AsyncIterable interface includes the properties described inTable 68:

Table 68: AsyncIterable Interface Required Properties
PropertyValueRequirements
@@asyncIteratorA function that returns an AsyncIterator object.The returned object must conform to the AsyncIterator interface.

27.1.1.4 The AsyncIterator Interface

An object that implements the AsyncIterator interface must include the properties inTable 69. Such objects may also implement the properties inTable 70.

Table 69: AsyncIterator Interface Required Properties
PropertyValueRequirements
"next"A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. If a previous call to the next method of an AsyncIterator has returned a promise for an IteratorResult object whose"done"property istrue, then all subsequent calls to the next method of that object should also return a promise for an IteratorResult object whose"done"property istrue. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a"value"property whose value is not a promise (or "thenable"). However, this requirement is also not enforced.

Note 1

Arguments may be passed to the next function but their interpretation and validity is dependent upon the target AsyncIterator. The for-await-of statement and other common users of AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be used in such a manner must be prepared to deal with being called with no arguments.

Table 70: AsyncIterator Interface Optional Properties
PropertyValueRequirements
"return"A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller does not intend to make any more next method calls to the AsyncIterator. The returned promise will fulfill with an IteratorResult object which will typically have a"done"property whose value istrue, and a"value"property with the value passed as the argument of the return method. However, this requirement is not enforced.

Additionally, the IteratorResult object that serves as a fulfillment value should have a"value"property whose value is not a promise (or "thenable"). If the argument value is used in the typical manner, then if it is a rejected promise, a promise rejected with the same reason should be returned; if it is a fulfilled promise, then its fulfillment value should be used as the"value"property of the returned promise's IteratorResult object fulfillment value. However, these requirements are also not enforced.

"throw"A function that returns a promise for an IteratorResult object.

The returned promise, when fulfilled, must fulfill with an object which conforms to the IteratorResult interface. Invoking this method notifies the AsyncIterator object that the caller has detected an error condition. The argument may be used to identify the error condition and typically will be an exception object. A typical response is to return a rejected promise which rejects with the value passed as the argument.

If the returned promise is fulfilled, the IteratorResult fulfillment value will typically have a"done"property whose value istrue. Additionally, it should have a"value"property whose value is not a promise (or "thenable"), but this requirement is not enforced.

Note 2

Typically callers of these methods should check for their existence before invoking them. Certain ECMAScript language features including for-await-of and yield* call these methods after performing an existence check.

27.1.1.5 The IteratorResult Interface

The IteratorResult interface includes the properties listed inTable 71:

Table 71: IteratorResult Interface Properties
PropertyValueRequirements
"done"Eithertrueorfalse.This is the result status of an iterator next method call. If the end of the iterator was reached"done"istrue. If the end was not reached"done"isfalseand a value is available. If a"done"property (either own or inherited) does not exist, it is considered to have the valuefalse.
"value"AnyECMAScript language value.If done isfalse, this is the current iteration element value. If done istrue, this is the return value of the iterator, if it supplied one. If the iterator does not have a return value,"value"isundefined. In that case, the"value"property may be absent from the conforming object if it does not inherit an explicit"value"property.

27.1.2 The %IteratorPrototype% Object

The %IteratorPrototype% object:

Note

All objects defined in this specification that implement the Iterator interface also inherit from %IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The %IteratorPrototype% object provides a place where additional methods that are applicable to all iterator objects may be added.

The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:

Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()))

27.1.2.1 %IteratorPrototype% [ @@iterator ] ( )

The following steps are taken:

  1. Return thethisvalue.

The value of the"name"property of this function is"[Symbol.iterator]".

27.1.3 The %AsyncIteratorPrototype% Object

The %AsyncIteratorPrototype% object:

Note

All objects defined in this specification that implement the AsyncIterator interface also inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods that are applicable to all async iterator objects may be added.

27.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )

The following steps are taken:

  1. Return thethisvalue.

The value of the"name"property of this function is"[Symbol.asyncIterator]".

27.1.4 Async-from-Sync Iterator Objects

An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There is not a namedconstructorfor Async-from-Sync Iterator objects. Instead, Async-from-Sync iterator objects are created by theCreateAsyncFromSyncIteratorabstract operation as needed.

27.1.4.1 CreateAsyncFromSyncIterator ( syncIteratorRecord )

The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord. It is used to create an async iteratorRecordfrom a synchronous iteratorRecord. It performs the following steps when called:

  1. Let asyncIterator be ! OrdinaryObjectCreate(%AsyncFromSyncIteratorPrototype%, « [[SyncIteratorRecord]] »).
  2. Set asyncIterator.[[SyncIteratorRecord]] to syncIteratorRecord.
  3. Let nextMethod be ! Get(asyncIterator,"next").
  4. Let iteratorRecord be theRecord{ [[Iterator]]: asyncIterator, [[NextMethod]]: nextMethod, [[Done]]:false}.
  5. Return iteratorRecord.

27.1.4.2 The %AsyncFromSyncIteratorPrototype% Object

The %AsyncFromSyncIteratorPrototype% object:

  • has properties that are inherited by all Async-from-Sync Iterator Objects.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%AsyncIteratorPrototype%.
  • has the following properties:

27.1.4.2.1 %AsyncFromSyncIteratorPrototype%.next ( [ value ] )

  1. Let O be thethisvalue.
  2. Assert:Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIteratorRecord be O.[[SyncIteratorRecord]].
  5. If value is present, then
    1. Let result beIteratorNext(syncIteratorRecord, value).
  6. Else,
    1. Let result beIteratorNext(syncIteratorRecord).
  7. IfAbruptRejectPromise(result, promiseCapability).
  8. Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.2.2 %AsyncFromSyncIteratorPrototype%.return ( [ value ] )

  1. Let O be thethisvalue.
  2. Assert:Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
  5. Let return beGetMethod(syncIterator,"return").
  6. IfAbruptRejectPromise(return, promiseCapability).
  7. If return isundefined, then
    1. Let iterResult be ! CreateIterResultObject(value,true).
    2. Perform ! Call(promiseCapability.[[Resolve]],undefined, « iterResult »).
    3. Return promiseCapability.[[Promise]].
  8. If value is present, then
    1. Let result beCall(return, syncIterator, « value »).
  9. Else,
    1. Let result beCall(return, syncIterator).
  10. IfAbruptRejectPromise(result, promiseCapability).
  11. IfType(result) is not Object, then
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « a newly createdTypeErrorobject »).
    2. Return promiseCapability.[[Promise]].
  12. Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.2.3 %AsyncFromSyncIteratorPrototype%.throw ( [ value ] )

Note
In this specification, value is always provided, but is left optional for consistency with%AsyncFromSyncIteratorPrototype%.return ( [ value ] ).
  1. Let O be thethisvalue.
  2. Assert:Type(O) is Object and O has a [[SyncIteratorRecord]] internal slot.
  3. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  4. Let syncIterator be O.[[SyncIteratorRecord]].[[Iterator]].
  5. Let throw beGetMethod(syncIterator,"throw").
  6. IfAbruptRejectPromise(throw, promiseCapability).
  7. If throw isundefined, then
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « value »).
    2. Return promiseCapability.[[Promise]].
  8. If value is present, then
    1. Let result beCall(throw, syncIterator, « value »).
  9. Else,
    1. Let result beCall(throw, syncIterator).
  10. IfAbruptRejectPromise(result, promiseCapability).
  11. IfType(result) is not Object, then
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « a newly createdTypeErrorobject »).
    2. Return promiseCapability.[[Promise]].
  12. Return ! AsyncFromSyncIteratorContinuation(result, promiseCapability).

27.1.4.3 Properties of Async-from-Sync Iterator Instances

Async-from-Sync Iterator instances are ordinary objects that inherit properties from the%AsyncFromSyncIteratorPrototype%intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed inTable 72. Async-from-Sync Iterator instances are not directly observable from ECMAScript code.

Table 72: Internal Slots of Async-from-Sync Iterator Instances
Internal SlotDescription
[[SyncIteratorRecord]]ARecord, of the type returned byGetIterator, representing the original synchronous iterator which is being adapted.

27.1.4.4 AsyncFromSyncIteratorContinuation ( result, promiseCapability )

The abstract operation AsyncFromSyncIteratorContinuation takes arguments result and promiseCapability (aPromiseCapability Record). It performs the following steps when called:

  1. Let done beIteratorComplete(result).
  2. IfAbruptRejectPromise(done, promiseCapability).
  3. Let value beIteratorValue(result).
  4. IfAbruptRejectPromise(value, promiseCapability).
  5. Let valueWrapper bePromiseResolve(%Promise%, value).
  6. IfAbruptRejectPromise(valueWrapper, promiseCapability).
  7. Let unwrap be a newAbstract Closurewith parameters (value) that captures done and performs the following steps when called:
    1. Return ! CreateIterResultObject(value, done).
  8. Let onFulfilled be ! CreateBuiltinFunction(unwrap, 1,"", « »).
  9. NOTE: onFulfilled is used when processing the"value"property of an IteratorResult object in order to wait for its value if it is a promise and re-package the result in a new "unwrapped" IteratorResult object.
  10. Perform ! PerformPromiseThen(valueWrapper, onFulfilled,undefined, promiseCapability).
  11. Return promiseCapability.[[Promise]].

27.2 Promise Objects

A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly asynchronous) computation.

Any Promise object is in one of three mutually exclusive states: fulfilled, rejected, and pending:

  • A promise p is fulfilled if p.then(f, r) will immediately enqueue aJobto call the function f.
  • A promise p is rejected if p.then(f, r) will immediately enqueue aJobto call the function r.
  • A promise is pending if it is neither fulfilled nor rejected.

A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.

A promise is resolved if it is settled or if it has been “locked in” to match the state of another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending, fulfilled or rejected.

27.2.1 Promise Abstract Operations

27.2.1.1 PromiseCapability Records

A PromiseCapability Record is aRecordvalue used to encapsulate a promise object along with the functions that are capable of resolving or rejecting that promise object. PromiseCapability Records are produced by theNewPromiseCapabilityabstract operation.

PromiseCapability Records have the fields listed inTable 73.

Table 73:PromiseCapability RecordFields
Field NameValueMeaning
[[Promise]]An objectAn object that is usable as a promise.
[[Resolve]]Afunction objectThe function that is used to resolve the given promise object.
[[Reject]]Afunction objectThe function that is used to reject the given promise object.

27.2.1.1.1 IfAbruptRejectPromise ( value, capability )

IfAbruptRejectPromise is a shorthand for a sequence of algorithm steps that use aPromiseCapability Record. An algorithm step of the form:

  1. IfAbruptRejectPromise(value, capability).

means the same thing as:

  1. If value is anabrupt completion, then
    1. Perform ? Call(capability.[[Reject]],undefined, « value.[[Value]] »).
    2. Return capability.[[Promise]].
  2. Else if value is aCompletion Record, set value to value.[[Value]].

27.2.1.2 PromiseReaction Records

The PromiseReaction is aRecordvalue used to store information about how a promise should react when it becomes resolved or rejected with a given value. PromiseReaction records are created by thePerformPromiseThenabstract operation, and are used by theAbstract Closurereturned byNewPromiseReactionJob.

PromiseReaction records have the fields listed inTable 74.

Table 74: PromiseReactionRecordFields
Field NameValueMeaning
[[Capability]]APromiseCapability Record, orundefinedThe capabilities of the promise for which this record provides a reaction handler.
[[Type]]Fulfill|RejectThe [[Type]] is used when [[Handler]] isemptyto allow for behaviour specific to the settlement type.
[[Handler]]AJobCallback Record|empty.The function that should be applied to the incoming value, and whose return value will govern what happens to the derived promise. If [[Handler]] isempty, a function that depends on the value of [[Type]] will be used instead.

27.2.1.3 CreateResolvingFunctions ( promise )

The abstract operation CreateResolvingFunctions takes argument promise. It performs the following steps when called:

  1. Let alreadyResolved be theRecord{ [[Value]]:false}.
  2. Let stepsResolve be the algorithm steps defined inPromise Resolve Functions.
  3. Let lengthResolve be the number of non-optional parameters of the function definition inPromise Resolve Functions.
  4. Let resolve be ! CreateBuiltinFunction(stepsResolve, lengthResolve,"", « [[Promise]], [[AlreadyResolved]] »).
  5. Set resolve.[[Promise]] to promise.
  6. Set resolve.[[AlreadyResolved]] to alreadyResolved.
  7. Let stepsReject be the algorithm steps defined inPromise Reject Functions.
  8. Let lengthReject be the number of non-optional parameters of the function definition inPromise Reject Functions.
  9. Let reject be ! CreateBuiltinFunction(stepsReject, lengthReject,"", « [[Promise]], [[AlreadyResolved]] »).
  10. Set reject.[[Promise]] to promise.
  11. Set reject.[[AlreadyResolved]] to alreadyResolved.
  12. Return theRecord{ [[Resolve]]: resolve, [[Reject]]: reject }.

27.2.1.3.1 Promise Reject Functions

A promise reject function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise reject function is called with argument reason, the following steps are taken:

  1. Let F be theactive function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] istrue, returnundefined.
  6. Set alreadyResolved.[[Value]] totrue.
  7. ReturnRejectPromise(promise, reason).

The"length"property of a promise reject function is1𝔽.

27.2.1.3.2 Promise Resolve Functions

A promise resolve function is an anonymous built-in function that has [[Promise]] and [[AlreadyResolved]] internal slots.

When a promise resolve function is called with argument resolution, the following steps are taken:

  1. Let F be theactive function object.
  2. Assert: F has a [[Promise]] internal slot whose value is an Object.
  3. Let promise be F.[[Promise]].
  4. Let alreadyResolved be F.[[AlreadyResolved]].
  5. If alreadyResolved.[[Value]] istrue, returnundefined.
  6. Set alreadyResolved.[[Value]] totrue.
  7. IfSameValue(resolution, promise) istrue, then
    1. Let selfResolutionError be a newly createdTypeErrorobject.
    2. ReturnRejectPromise(promise, selfResolutionError).
  8. IfType(resolution) is not Object, then
    1. ReturnFulfillPromise(promise, resolution).
  9. Let then beGet(resolution,"then").
  10. If then is anabrupt completion, then
    1. ReturnRejectPromise(promise, then.[[Value]]).
  11. Let thenAction be then.[[Value]].
  12. IfIsCallable(thenAction) isfalse, then
    1. ReturnFulfillPromise(promise, resolution).
  13. Let thenJobCallback beHostMakeJobCallback(thenAction).
  14. Let job beNewPromiseResolveThenableJob(promise, resolution, thenJobCallback).
  15. PerformHostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  16. Returnundefined.

The"length"property of a promise resolve function is1𝔽.

27.2.1.4 FulfillPromise ( promise, value )

The abstract operation FulfillPromise takes arguments promise and value. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] ispending.
  2. Let reactions be promise.[[PromiseFulfillReactions]].
  3. Set promise.[[PromiseResult]] to value.
  4. Set promise.[[PromiseFulfillReactions]] toundefined.
  5. Set promise.[[PromiseRejectReactions]] toundefined.
  6. Set promise.[[PromiseState]] tofulfilled.
  7. ReturnTriggerPromiseReactions(reactions, value).

27.2.1.5 NewPromiseCapability ( C )

The abstract operation NewPromiseCapability takes argument C. It attempts to use C as aconstructorin the fashion of the built-in Promiseconstructorto create a Promise object and extract its resolve and reject functions. The Promise object plus the resolve and reject functions are used to initialize a newPromiseCapability Record. It performs the following steps when called:

  1. IfIsConstructor(C) isfalse, throw aTypeErrorexception.
  2. NOTE: C is assumed to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor(see27.2.3.1).
  3. Let promiseCapability be thePromiseCapability Record{ [[Promise]]:undefined, [[Resolve]]:undefined, [[Reject]]:undefined}.
  4. Let executorClosure be a newAbstract Closurewith parameters (resolve, reject) that captures promiseCapability and performs the following steps when called:
    1. If promiseCapability.[[Resolve]] is notundefined, throw aTypeErrorexception.
    2. If promiseCapability.[[Reject]] is notundefined, throw aTypeErrorexception.
    3. Set promiseCapability.[[Resolve]] to resolve.
    4. Set promiseCapability.[[Reject]] to reject.
    5. Returnundefined.
  5. Let executor be ! CreateBuiltinFunction(executorClosure, 2,"", « »).
  6. Let promise be ? Construct(C, « executor »).
  7. IfIsCallable(promiseCapability.[[Resolve]]) isfalse, throw aTypeErrorexception.
  8. IfIsCallable(promiseCapability.[[Reject]]) isfalse, throw aTypeErrorexception.
  9. Set promiseCapability.[[Promise]] to promise.
  10. Return promiseCapability.
Note

This abstract operation supports Promise subclassing, as it is generic on anyconstructorthat calls a passed executor function argument in the same way as the Promiseconstructor. It is used to generalize static methods of the Promiseconstructorto any subclass.

27.2.1.6 IsPromise ( x )

The abstract operation IsPromise takes argument x. It checks for the promise brand on an object. It performs the following steps when called:

  1. IfType(x) is not Object, returnfalse.
  2. If x does not have a [[PromiseState]] internal slot, returnfalse.
  3. Returntrue.

27.2.1.7 RejectPromise ( promise, reason )

The abstract operation RejectPromise takes arguments promise and reason. It performs the following steps when called:

  1. Assert: The value of promise.[[PromiseState]] ispending.
  2. Let reactions be promise.[[PromiseRejectReactions]].
  3. Set promise.[[PromiseResult]] to reason.
  4. Set promise.[[PromiseFulfillReactions]] toundefined.
  5. Set promise.[[PromiseRejectReactions]] toundefined.
  6. Set promise.[[PromiseState]] torejected.
  7. If promise.[[PromiseIsHandled]] isfalse, performHostPromiseRejectionTracker(promise,"reject").
  8. ReturnTriggerPromiseReactions(reactions, reason).

27.2.1.8 TriggerPromiseReactions ( reactions, argument )

The abstract operation TriggerPromiseReactions takes arguments reactions (aListof PromiseReaction Records) and argument. It enqueues a newJobfor each record in reactions. Each suchJobprocesses the [[Type]] and [[Handler]] of the PromiseReactionRecord, and if the [[Handler]] is notempty, calls it passing the given argument. If the [[Handler]] isempty, the behaviour is determined by the [[Type]]. It performs the following steps when called:

  1. For each element reaction of reactions, do
    1. Let job beNewPromiseReactionJob(reaction, argument).
    2. PerformHostEnqueuePromiseJob(job.[[Job]], job.[[Realm]]).
  2. Returnundefined.

27.2.1.9 HostPromiseRejectionTracker ( promise, operation )

Thehost-definedabstract operation HostPromiseRejectionTracker takes arguments promise (a Promise) and operation ("reject"or"handle"). It allowshostenvironments to track promise rejections.

An implementation of HostPromiseRejectionTracker must conform to the following requirements:

The default implementation of HostPromiseRejectionTracker is to returnNormalCompletion(empty).

Note 1

HostPromiseRejectionTracker is called in two scenarios:

  • When a promise is rejected without any handlers, it is called with its operation argument set to"reject".
  • When a handler is added to a rejected promise for the first time, it is called with its operation argument set to"handle".

A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled rejections, while also being careful to notify them if such previous notifications are later invalidated by new handlers being attached.

Note 2

If operation is"handle", an implementation should not hold a reference to promise in a way that would interfere with garbage collection. An implementation may hold a reference to promise if operation is"reject", since it is expected that rejections will be rare and not on hot code paths.

27.2.2 Promise Jobs

27.2.2.1 NewPromiseReactionJob ( reaction, argument )

The abstract operation NewPromiseReactionJob takes arguments reaction and argument. It returns a newJobAbstract Closurethat applies the appropriate handler to the incoming value, and uses the handler's return value to resolve or reject the derived promise associated with that handler. It performs the following steps when called:

  1. Let job be a newJobAbstract Closurewith no parameters that captures reaction and argument and performs the following steps when called:
    1. Assert: reaction is a PromiseReactionRecord.
    2. Let promiseCapability be reaction.[[Capability]].
    3. Let type be reaction.[[Type]].
    4. Let handler be reaction.[[Handler]].
    5. If handler isempty, then
      1. If type isFulfill, let handlerResult beNormalCompletion(argument).
      2. Else,
        1. Assert: type isReject.
        2. Let handlerResult beThrowCompletion(argument).
    6. Else, let handlerResult beHostCallJobCallback(handler,undefined, « argument »).
    7. If promiseCapability isundefined, then
      1. Assert: handlerResult is not anabrupt completion.
      2. ReturnNormalCompletion(empty).
    8. Assert: promiseCapability is aPromiseCapability Record.
    9. If handlerResult is anabrupt completion, then
      1. Let status beCall(promiseCapability.[[Reject]],undefined, « handlerResult.[[Value]] »).
    10. Else,
      1. Let status beCall(promiseCapability.[[Resolve]],undefined, « handlerResult.[[Value]] »).
    11. ReturnCompletion(status).
  2. Let handlerRealm benull.
  3. If reaction.[[Handler]] is notempty, then
    1. Let getHandlerRealmResult beGetFunctionRealm(reaction.[[Handler]].[[Callback]]).
    2. If getHandlerRealmResult is anormal completion, set handlerRealm to getHandlerRealmResult.[[Value]].
    3. Else, set handlerRealm tothe current Realm Record.
    4. NOTE: handlerRealm is nevernullunless the handler isundefined. When the handler is a revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error objects.
  4. Return theRecord{ [[Job]]: job, [[Realm]]: handlerRealm }.

27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve, thenable, then )

The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve, thenable, and then. It performs the following steps when called:

  1. Let job be a newJobAbstract Closurewith no parameters that captures promiseToResolve, thenable, and then and performs the following steps when called:
    1. Let resolvingFunctions beCreateResolvingFunctions(promiseToResolve).
    2. Let thenCallResult beHostCallJobCallback(then, thenable, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
    3. If thenCallResult is anabrupt completion, then
      1. Let status beCall(resolvingFunctions.[[Reject]],undefined, « thenCallResult.[[Value]] »).
      2. ReturnCompletion(status).
    4. ReturnCompletion(thenCallResult).
  2. Let getThenRealmResult beGetFunctionRealm(then.[[Callback]]).
  3. If getThenRealmResult is anormal completion, let thenRealm be getThenRealmResult.[[Value]].
  4. Else, let thenRealm bethe current Realm Record.
  5. NOTE: thenRealm is nevernull. When then.[[Callback]] is a revoked Proxy and no code runs, thenRealm is used to create error objects.
  6. Return theRecord{ [[Job]]: job, [[Realm]]: thenRealm }.
Note

ThisJobuses the supplied thenable and its then method to resolve the given promise. This process must take place as aJobto ensure that the evaluation of the then method occurs after evaluation of any surrounding code has completed.

27.2.3 The Promise Constructor

The Promiseconstructor:

  • is %Promise%.
  • is the initial value of the"Promise"property of theglobal object.
  • creates and initializes a new Promise object when called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.
  • may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit the specified Promise behaviour must include a super call to the Promiseconstructorto create and initialize the subclass instance with the internal state necessary to support the Promise and Promise.prototype built-in methods.

27.2.3.1 Promise ( executor )

When the Promise function is called with argument executor, the following steps are taken:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. IfIsCallable(executor) isfalse, throw aTypeErrorexception.
  3. Let promise be ? OrdinaryCreateFromConstructor(NewTarget,"%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
  4. Set promise.[[PromiseState]] topending.
  5. Set promise.[[PromiseFulfillReactions]] to a new emptyList.
  6. Set promise.[[PromiseRejectReactions]] to a new emptyList.
  7. Set promise.[[PromiseIsHandled]] tofalse.
  8. Let resolvingFunctions beCreateResolvingFunctions(promise).
  9. Let completion beCall(executor,undefined, « resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]] »).
  10. If completion is anabrupt completion, then
    1. Perform ? Call(resolvingFunctions.[[Reject]],undefined, « completion.[[Value]] »).
  11. Return promise.
Note

The executor argument must be afunction object. It is called for initiating and reporting completion of the possibly deferred action represented by this Promise object. The executor is called with two arguments: resolve and reject. These are functions that may be used by the executor function to report eventual completion or failure of the deferred computation. Returning from the executor function does not mean that the deferred action has been completed but only that the request to eventually perform the deferred action has been accepted.

The resolve function that is passed to an executor function accepts a single argument. The executor code may eventually call the resolve function to indicate that it wishes to resolve the associated Promise object. The argument passed to the resolve function represents the eventual value of the deferred action and can be either the actual fulfillment value or another Promise object which will provide the value if it is fulfilled.

The reject function that is passed to an executor function accepts a single argument. The executor code may eventually call the reject function to indicate that the associated Promise is rejected and will never be fulfilled. The argument passed to the reject function is used as the rejection value of the promise. Typically it will be an Error object.

The resolve and reject functions passed to an executor function by the Promiseconstructorhave the capability to actually resolve and reject the associated promise. Subclasses may have differentconstructorbehaviour that passes in customized values for resolve and reject.

27.2.4 Properties of the Promise Constructor

The Promiseconstructor:

27.2.4.1 Promise.all ( iterable )

The all function returns a new promise which is fulfilled with an array of fulfillment values for the passed promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be thethisvalue.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve beGetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord beGetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result bePerformPromiseAll(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is anabrupt completion, then
    1. If iteratorRecord.[[Done]] isfalse, set result toIteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. ReturnCompletion(result).
Note

The all function requires itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.

27.2.4.1.1 GetPromiseResolve ( promiseConstructor )

The abstract operation GetPromiseResolve takes argument promiseConstructor. It performs the following steps when called:

  1. Assert:IsConstructor(promiseConstructor) istrue.
  2. Let promiseResolve be ? Get(promiseConstructor,"resolve").
  3. IfIsCallable(promiseResolve) isfalse, throw aTypeErrorexception.
  4. Return promiseResolve.

27.2.4.1.2 PerformPromiseAll ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAll takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert:IsConstructor(constructor) istrue.
  2. Assert:IsCallable(promiseResolve) istrue.
  3. Let values be a new emptyList.
  4. Let remainingElementsCount be theRecord{ [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, then
      1. Set iteratorRecord.[[Done]] totrue.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let valuesArray be ! CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]],undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue beIteratorValue(next).
    6. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    7. ReturnIfAbrupt(nextValue).
    8. Appendundefinedto values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let steps be the algorithm steps defined inPromise.all Resolve Element Functions.
    11. Let length be the number of non-optional parameters of the function definition inPromise.all Resolve Element Functions.
    12. Let onFulfilled be ! CreateBuiltinFunction(steps, length,"", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Set onFulfilled.[[AlreadyCalled]] tofalse.
    14. Set onFulfilled.[[Index]] to index.
    15. Set onFulfilled.[[Values]] to values.
    16. Set onFulfilled.[[Capability]] to resultCapability.
    17. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    19. Perform ? Invoke(nextPromise,"then", « onFulfilled, resultCapability.[[Reject]] »).
    20. Set index to index + 1.

27.2.4.1.3 Promise.all Resolve Element Functions

A Promise.all resolve element function is an anonymous built-in function that is used to resolve a specific Promise.all element. Each Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.all resolve element function is called with argument x, the following steps are taken:

  1. Let F be theactive function object.
  2. If F.[[AlreadyCalled]] istrue, returnundefined.
  3. Set F.[[AlreadyCalled]] totrue.
  4. Let index be F.[[Index]].
  5. Let values be F.[[Values]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set values[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]],undefined, « valuesArray »).
  11. Returnundefined.

The"length"property of a Promise.all resolve element function is1𝔽.

27.2.4.2 Promise.allSettled ( iterable )

The allSettled function returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be thethisvalue.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve beGetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord beGetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result bePerformPromiseAllSettled(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is anabrupt completion, then
    1. If iteratorRecord.[[Done]] isfalse, set result toIteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. ReturnCompletion(result).
Note

The allSettled function requires itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.

27.2.4.2.1 PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAllSettled takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: ! IsConstructor(constructor) istrue.
  2. Assert:IsCallable(promiseResolve) istrue.
  3. Let values be a new emptyList.
  4. Let remainingElementsCount be theRecord{ [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, then
      1. Set iteratorRecord.[[Done]] totrue.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let valuesArray be ! CreateArrayFromList(values).
        2. Perform ? Call(resultCapability.[[Resolve]],undefined, « valuesArray »).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue beIteratorValue(next).
    6. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    7. ReturnIfAbrupt(nextValue).
    8. Appendundefinedto values.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsFulfilled be the algorithm steps defined inPromise.allSettled Resolve Element Functions.
    11. Let lengthFulfilled be the number of non-optional parameters of the function definition inPromise.allSettled Resolve Element Functions.
    12. Let onFulfilled be ! CreateBuiltinFunction(stepsFulfilled, lengthFulfilled,"", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    13. Let alreadyCalled be theRecord{ [[Value]]:false}.
    14. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
    15. Set onFulfilled.[[Index]] to index.
    16. Set onFulfilled.[[Values]] to values.
    17. Set onFulfilled.[[Capability]] to resultCapability.
    18. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
    19. Let stepsRejected be the algorithm steps defined inPromise.allSettled Reject Element Functions.
    20. Let lengthRejected be the number of non-optional parameters of the function definition inPromise.allSettled Reject Element Functions.
    21. Let onRejected be ! CreateBuiltinFunction(stepsRejected, lengthRejected,"", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
    22. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
    23. Set onRejected.[[Index]] to index.
    24. Set onRejected.[[Values]] to values.
    25. Set onRejected.[[Capability]] to resultCapability.
    26. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    27. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    28. Perform ? Invoke(nextPromise,"then", « onFulfilled, onRejected »).
    29. Set index to index + 1.

27.2.4.2.2 Promise.allSettled Resolve Element Functions

A Promise.allSettled resolve element function is an anonymous built-in function that is used to resolve a specific Promise.allSettled element. Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled resolve element function is called with argument x, the following steps are taken:

  1. Let F be theactive function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] istrue, returnundefined.
  4. Set alreadyCalled.[[Value]] totrue.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj,"status","fulfilled").
  11. Perform ! CreateDataPropertyOrThrow(obj,"value", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]],undefined, « valuesArray »).
  15. Returnundefined.

The"length"property of a Promise.allSettled resolve element function is1𝔽.

27.2.4.2.3 Promise.allSettled Reject Element Functions

A Promise.allSettled reject element function is an anonymous built-in function that is used to reject a specific Promise.allSettled element. Each Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.allSettled reject element function is called with argument x, the following steps are taken:

  1. Let F be theactive function object.
  2. Let alreadyCalled be F.[[AlreadyCalled]].
  3. If alreadyCalled.[[Value]] istrue, returnundefined.
  4. Set alreadyCalled.[[Value]] totrue.
  5. Let index be F.[[Index]].
  6. Let values be F.[[Values]].
  7. Let promiseCapability be F.[[Capability]].
  8. Let remainingElementsCount be F.[[RemainingElements]].
  9. Let obj be ! OrdinaryObjectCreate(%Object.prototype%).
  10. Perform ! CreateDataPropertyOrThrow(obj,"status","rejected").
  11. Perform ! CreateDataPropertyOrThrow(obj,"reason", x).
  12. Set values[index] to obj.
  13. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  14. If remainingElementsCount.[[Value]] is 0, then
    1. Let valuesArray be ! CreateArrayFromList(values).
    2. Return ? Call(promiseCapability.[[Resolve]],undefined, « valuesArray »).
  15. Returnundefined.

The"length"property of a Promise.allSettled reject element function is1𝔽.

27.2.4.3 Promise.any ( iterable )

The any function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be thethisvalue.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve beGetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord beGetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result bePerformPromiseAny(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is anabrupt completion, then
    1. If iteratorRecord.[[Done]] isfalse, set result toIteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. ReturnCompletion(result).
Note

The any function requires itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.

27.2.4.3.1 PerformPromiseAny ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseAny takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert: ! IsConstructor(constructor) istrue.
  2. Assert: ! IsCallable(promiseResolve) istrue.
  3. Let errors be a new emptyList.
  4. Let remainingElementsCount be theRecord{ [[Value]]: 1 }.
  5. Let index be 0.
  6. Repeat,
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, then
      1. Set iteratorRecord.[[Done]] totrue.
      2. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
      3. If remainingElementsCount.[[Value]] is 0, then
        1. Let error be a newly createdAggregateErrorobject.
        2. Perform ! DefinePropertyOrThrow(error,"errors", PropertyDescriptor { [[Configurable]]:true, [[Enumerable]]:false, [[Writable]]:true, [[Value]]: ! CreateArrayFromList(errors) }).
        3. ReturnThrowCompletion(error).
      4. Return resultCapability.[[Promise]].
    5. Let nextValue beIteratorValue(next).
    6. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    7. ReturnIfAbrupt(nextValue).
    8. Appendundefinedto errors.
    9. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    10. Let stepsRejected be the algorithm steps defined inPromise.any Reject Element Functions.
    11. Let lengthRejected be the number of non-optional parameters of the function definition inPromise.any Reject Element Functions.
    12. Let onRejected be ! CreateBuiltinFunction(stepsRejected, lengthRejected,"", « [[AlreadyCalled]], [[Index]], [[Errors]], [[Capability]], [[RemainingElements]] »).
    13. Set onRejected.[[AlreadyCalled]] tofalse.
    14. Set onRejected.[[Index]] to index.
    15. Set onRejected.[[Errors]] to errors.
    16. Set onRejected.[[Capability]] to resultCapability.
    17. Set onRejected.[[RemainingElements]] to remainingElementsCount.
    18. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
    19. Perform ? Invoke(nextPromise,"then", « resultCapability.[[Resolve]], onRejected »).
    20. Set index to index + 1.

27.2.4.3.2 Promise.any Reject Element Functions

A Promise.any reject element function is an anonymous built-in function that is used to reject a specific Promise.any element. Each Promise.any reject element function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.

When a Promise.any reject element function is called with argument x, the following steps are taken:

  1. Let F be theactive function object.
  2. If F.[[AlreadyCalled]] istrue, returnundefined.
  3. Set F.[[AlreadyCalled]] totrue.
  4. Let index be F.[[Index]].
  5. Let errors be F.[[Errors]].
  6. Let promiseCapability be F.[[Capability]].
  7. Let remainingElementsCount be F.[[RemainingElements]].
  8. Set errors[index] to x.
  9. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] - 1.
  10. If remainingElementsCount.[[Value]] is 0, then
    1. Let error be a newly createdAggregateErrorobject.
    2. Perform ! DefinePropertyOrThrow(error,"errors", PropertyDescriptor { [[Configurable]]:true, [[Enumerable]]:false, [[Writable]]:true, [[Value]]: ! CreateArrayFromList(errors) }).
    3. Return ? Call(promiseCapability.[[Reject]],undefined, « error »).
  11. Returnundefined.

The"length"property of a Promise.any reject element function is1𝔽.

27.2.4.4 Promise.prototype

The initial value of Promise.prototype is thePromise prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

27.2.4.5 Promise.race ( iterable )

The race function returns a new promise which is settled in the same way as the first passed promise to settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.

  1. Let C be thethisvalue.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Let promiseResolve beGetPromiseResolve(C).
  4. IfAbruptRejectPromise(promiseResolve, promiseCapability).
  5. Let iteratorRecord beGetIterator(iterable).
  6. IfAbruptRejectPromise(iteratorRecord, promiseCapability).
  7. Let result bePerformPromiseRace(iteratorRecord, C, promiseCapability, promiseResolve).
  8. If result is anabrupt completion, then
    1. If iteratorRecord.[[Done]] isfalse, set result toIteratorClose(iteratorRecord, result).
    2. IfAbruptRejectPromise(result, promiseCapability).
  9. ReturnCompletion(result).
Note 1

If the iterable argument is empty or if none of the promises in iterable ever settle then the pending promise returned by this method will never be settled.

Note 2

The race function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor. It also expects that itsthisvalue provides a resolve method.

27.2.4.5.1 PerformPromiseRace ( iteratorRecord, constructor, resultCapability, promiseResolve )

The abstract operation PerformPromiseRace takes arguments iteratorRecord, constructor, resultCapability (aPromiseCapability Record), and promiseResolve. It performs the following steps when called:

  1. Assert:IsConstructor(constructor) istrue.
  2. Assert:IsCallable(promiseResolve) istrue.
  3. Repeat,
    1. Let next beIteratorStep(iteratorRecord).
    2. If next is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    3. ReturnIfAbrupt(next).
    4. If next isfalse, then
      1. Set iteratorRecord.[[Done]] totrue.
      2. Return resultCapability.[[Promise]].
    5. Let nextValue beIteratorValue(next).
    6. If nextValue is anabrupt completion, set iteratorRecord.[[Done]] totrue.
    7. ReturnIfAbrupt(nextValue).
    8. Let nextPromise be ? Call(promiseResolve, constructor, « nextValue »).
    9. Perform ? Invoke(nextPromise,"then", « resultCapability.[[Resolve]], resultCapability.[[Reject]] »).

27.2.4.6 Promise.reject ( r )

The reject function returns a new promise rejected with the passed argument.

  1. Let C be thethisvalue.
  2. Let promiseCapability be ? NewPromiseCapability(C).
  3. Perform ? Call(promiseCapability.[[Reject]],undefined, « r »).
  4. Return promiseCapability.[[Promise]].
Note

The reject function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.

27.2.4.7 Promise.resolve ( x )

The resolve function returns either a new promise resolved with the passed argument, or the argument itself if the argument is a promise produced by thisconstructor.

  1. Let C be thethisvalue.
  2. IfType(C) is not Object, throw aTypeErrorexception.
  3. Return ? PromiseResolve(C, x).
Note

The resolve function expects itsthisvalue to be aconstructorfunction that supports the parameter conventions of the Promiseconstructor.

27.2.4.7.1 PromiseResolve ( C, x )

The abstract operation PromiseResolve takes arguments C (aconstructor) and x (anECMAScript language value). It returns a new promise resolved with x. It performs the following steps when called:

  1. Assert:Type(C) is Object.
  2. IfIsPromise(x) istrue, then
    1. Let xConstructor be ? Get(x,"constructor").
    2. IfSameValue(xConstructor, C) istrue, return x.
  3. Let promiseCapability be ? NewPromiseCapability(C).
  4. Perform ? Call(promiseCapability.[[Resolve]],undefined, « x »).
  5. Return promiseCapability.[[Promise]].

27.2.4.8 get Promise [ @@species ]

Promise[@@species] is anaccessor propertywhose set accessor function isundefined. Its get accessor function performs the following steps:

  1. Return thethisvalue.

The value of the"name"property of this function is"get [Symbol.species]".

Note

Promise prototype methods normally use theirthisvalue'sconstructorto create a derived object. However, a subclassconstructormay over-ride that default behaviour by redefining its@@speciesproperty.

27.2.5 Properties of the Promise Prototype Object

The Promise prototype object:

  • is %Promise.prototype%.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is anordinary object.
  • does not have a [[PromiseState]] internal slot or any of the other internal slots of Promise instances.

27.2.5.1 Promise.prototype.catch ( onRejected )

When the catch method is called with argument onRejected, the following steps are taken:

  1. Let promise be thethisvalue.
  2. Return ? Invoke(promise,"then", «undefined, onRejected »).

27.2.5.2 Promise.prototype.constructor

The initial value of Promise.prototype.constructor is%Promise%.

27.2.5.3 Promise.prototype.finally ( onFinally )

When the finally method is called with argument onFinally, the following steps are taken:

  1. Let promise be thethisvalue.
  2. IfType(promise) is not Object, throw aTypeErrorexception.
  3. Let C be ? SpeciesConstructor(promise,%Promise%).
  4. Assert:IsConstructor(C) istrue.
  5. IfIsCallable(onFinally) isfalse, then
    1. Let thenFinally be onFinally.
    2. Let catchFinally be onFinally.
  6. Else,
    1. Let thenFinallyClosure be a newAbstract Closurewith parameters (value) that captures onFinally and C and performs the following steps when called:
      1. Let result be ? Call(onFinally,undefined).
      2. Let promise be ? PromiseResolve(C, result).
      3. Let returnValue be a newAbstract Closurewith no parameters that captures value and performs the following steps when called:
        1. Return value.
      4. Let valueThunk be ! CreateBuiltinFunction(returnValue, 0,"", « »).
      5. Return ? Invoke(promise,"then", « valueThunk »).
    2. Let thenFinally be ! CreateBuiltinFunction(thenFinallyClosure, 1,"", « »).
    3. Let catchFinallyClosure be a newAbstract Closurewith parameters (reason) that captures onFinally and C and performs the following steps when called:
      1. Let result be ? Call(onFinally,undefined).
      2. Let promise be ? PromiseResolve(C, result).
      3. Let throwReason be a newAbstract Closurewith no parameters that captures reason and performs the following steps when called:
        1. ReturnThrowCompletion(reason).
      4. Let thrower be ! CreateBuiltinFunction(throwReason, 0,"", « »).
      5. Return ? Invoke(promise,"then", « thrower »).
    4. Let catchFinally be ! CreateBuiltinFunction(catchFinallyClosure, 1,"", « »).
  7. Return ? Invoke(promise,"then", « thenFinally, catchFinally »).

27.2.5.4 Promise.prototype.then ( onFulfilled, onRejected )

When the then method is called with arguments onFulfilled and onRejected, the following steps are taken:

  1. Let promise be thethisvalue.
  2. IfIsPromise(promise) isfalse, throw aTypeErrorexception.
  3. Let C be ? SpeciesConstructor(promise,%Promise%).
  4. Let resultCapability be ? NewPromiseCapability(C).
  5. ReturnPerformPromiseThen(promise, onFulfilled, onRejected, resultCapability).

27.2.5.4.1 PerformPromiseThen ( promise, onFulfilled, onRejected [ , resultCapability ] )

The abstract operation PerformPromiseThen takes arguments promise, onFulfilled, and onRejected and optional argument resultCapability (aPromiseCapability Record). It performs the “then” operation on promise using onFulfilled and onRejected as its settlement actions. If resultCapability is passed, the result is stored by updating resultCapability's promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal operation where the result does not matter. It performs the following steps when called:

  1. Assert:IsPromise(promise) istrue.
  2. If resultCapability is not present, then
    1. Set resultCapability toundefined.
  3. IfIsCallable(onFulfilled) isfalse, then
    1. Let onFulfilledJobCallback beempty.
  4. Else,
    1. Let onFulfilledJobCallback beHostMakeJobCallback(onFulfilled).
  5. IfIsCallable(onRejected) isfalse, then
    1. Let onRejectedJobCallback beempty.
  6. Else,
    1. Let onRejectedJobCallback beHostMakeJobCallback(onRejected).
  7. Let fulfillReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]:Fulfill, [[Handler]]: onFulfilledJobCallback }.
  8. Let rejectReaction be the PromiseReaction { [[Capability]]: resultCapability, [[Type]]:Reject, [[Handler]]: onRejectedJobCallback }.
  9. If promise.[[PromiseState]] ispending, then
    1. Append fulfillReaction as the last element of theListthat is promise.[[PromiseFulfillReactions]].
    2. Append rejectReaction as the last element of theListthat is promise.[[PromiseRejectReactions]].
  10. Else if promise.[[PromiseState]] isfulfilled, then
    1. Let value be promise.[[PromiseResult]].
    2. Let fulfillJob beNewPromiseReactionJob(fulfillReaction, value).
    3. PerformHostEnqueuePromiseJob(fulfillJob.[[Job]], fulfillJob.[[Realm]]).
  11. Else,
    1. Assert: The value of promise.[[PromiseState]] isrejected.
    2. Let reason be promise.[[PromiseResult]].
    3. If promise.[[PromiseIsHandled]] isfalse, performHostPromiseRejectionTracker(promise,"handle").
    4. Let rejectJob beNewPromiseReactionJob(rejectReaction, reason).
    5. PerformHostEnqueuePromiseJob(rejectJob.[[Job]], rejectJob.[[Realm]]).
  12. Set promise.[[PromiseIsHandled]] totrue.
  13. If resultCapability isundefined, then
    1. Returnundefined.
  14. Else,
    1. Return resultCapability.[[Promise]].

27.2.5.5 Promise.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Promise".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.2.6 Properties of Promise Instances

Promise instances are ordinary objects that inherit properties from thePromise prototype object(the intrinsic,%Promise.prototype%). Promise instances are initially created with the internal slots described inTable 75.

Table 75: Internal Slots of Promise Instances
Internal SlotDescription
[[PromiseState]]One ofpending,fulfilled, orrejected. Governs how a promise will react to incoming calls to its then method.
[[PromiseResult]]The value with which the promise has been fulfilled or rejected, if any. Only meaningful if [[PromiseState]] is notpending.
[[PromiseFulfillReactions]]AListof PromiseReaction records to be processed when/if the promise transitions from thependingstate to thefulfilledstate.
[[PromiseRejectReactions]]AListof PromiseReaction records to be processed when/if the promise transitions from thependingstate to therejectedstate.
[[PromiseIsHandled]]A boolean indicating whether the promise has ever had a fulfillment or rejection handler; used in unhandled rejection tracking.

27.3 GeneratorFunction Objects

GeneratorFunction objects are functions that are usually created by evaluatingGeneratorDeclarations,GeneratorExpressions, andGeneratorMethods. They may also be created by calling the%GeneratorFunction%intrinsic.

Figure 5 (Informative): Generator Objects Relationships
A staggering variety of boxes and arrows.

27.3.1 The GeneratorFunction Constructor

The GeneratorFunctionconstructor:

  • is %GeneratorFunction%.
  • is a subclass of Function.
  • creates and initializes a new GeneratorFunction object when called as a function rather than as aconstructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunctionconstructorto create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.

27.3.1.1 GeneratorFunction ( p1, p2, … , pn, body )

The last argument specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.

When the GeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no “p” arguments, and where body might also not be provided), the following steps are taken:

  1. Let C be theactive function object.
  2. Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
  3. Return ? CreateDynamicFunction(C, NewTarget,generator, args).
Note

See NOTE for20.2.1.1.

27.3.2 Properties of the GeneratorFunction Constructor

The GeneratorFunctionconstructor:

  • is a standard built-infunction objectthat inherits from the Functionconstructor.
  • has a [[Prototype]] internal slot whose value is%Function%.
  • has a"name"property whose value is"GeneratorFunction".
  • has the following properties:

27.3.2.1 GeneratorFunction.length

This is adata propertywith a value of 1. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.3.2.2 GeneratorFunction.prototype

The initial value of GeneratorFunction.prototype is theGeneratorFunction prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

27.3.3 Properties of the GeneratorFunction Prototype Object

The GeneratorFunction prototype object:

27.3.3.1 GeneratorFunction.prototype.constructor

The initial value of GeneratorFunction.prototype.constructor is%GeneratorFunction%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.3.3.2 GeneratorFunction.prototype.prototype

The initial value of GeneratorFunction.prototype.prototype is theGenerator prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"GeneratorFunction".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.3.4 GeneratorFunction Instances

Every GeneratorFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse.

Each GeneratorFunction instance has the following own properties:

27.3.4.1 length

The specification for the"length"property of Function instances given in20.2.4.1also applies to GeneratorFunction instances.

27.3.4.2 name

The specification for the"name"property of Function instances given in20.2.4.2also applies to GeneratorFunction instances.

27.3.4.3 prototype

Whenever a GeneratorFunction instance is created anotherordinary objectis also created and is the initial value of the generator function's"prototype"property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator object when the generatorfunction objectis invoked using [[Call]].

This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.

Note

Unlike Function instances, the object that is the value of the a GeneratorFunction's"prototype"property does not have a"constructor"property whose value is the GeneratorFunction instance.

27.4 AsyncGeneratorFunction Objects

AsyncGeneratorFunction objects are functions that are usually created by evaluatingAsyncGeneratorDeclaration,AsyncGeneratorExpression, andAsyncGeneratorMethodsyntactic productions. They may also be created by calling the%AsyncGeneratorFunction%intrinsic.

27.4.1 The AsyncGeneratorFunction Constructor

The AsyncGeneratorFunctionconstructor:

  • is %AsyncGeneratorFunction%.
  • is a subclass of Function.
  • creates and initializes a new AsyncGeneratorFunction object when called as a function rather than as aconstructor. Thus the function call AsyncGeneratorFunction (...) is equivalent to the object creation expression new AsyncGeneratorFunction (...) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncGeneratorFunction behaviour must include a super call to the AsyncGeneratorFunctionconstructorto create and initialize subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All ECMAScript syntactic forms for defining async generator function objects create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of AsyncGeneratorFunction subclasses.

27.4.1.1 AsyncGeneratorFunction ( p1, p2, … , pn, body )

The last argument specifies the body (executable code) of an async generator function; any preceding arguments specify formal parameters.

When the AsyncGeneratorFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no "p" arguments, and where body might also not be provided), the following steps are taken:

  1. Let C be theactive function object.
  2. Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
  3. Return ? CreateDynamicFunction(C, NewTarget,asyncGenerator, args).
Note

See NOTE for20.2.1.1.

27.4.2 Properties of the AsyncGeneratorFunction Constructor

The AsyncGeneratorFunctionconstructor:

  • is a standard built-infunction objectthat inherits from the Functionconstructor.
  • has a [[Prototype]] internal slot whose value is%Function%.
  • has a"name"property whose value is"AsyncGeneratorFunction".
  • has the following properties:

27.4.2.1 AsyncGeneratorFunction.length

This is adata propertywith a value of 1. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.4.2.2 AsyncGeneratorFunction.prototype

The initial value of AsyncGeneratorFunction.prototype is theAsyncGeneratorFunction prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

27.4.3 Properties of the AsyncGeneratorFunction Prototype Object

The AsyncGeneratorFunction prototype object:

27.4.3.1 AsyncGeneratorFunction.prototype.constructor

The initial value of AsyncGeneratorFunction.prototype.constructor is%AsyncGeneratorFunction%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.4.3.2 AsyncGeneratorFunction.prototype.prototype

The initial value of AsyncGeneratorFunction.prototype.prototype is theAsyncGenerator prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.4.3.3 AsyncGeneratorFunction.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"AsyncGeneratorFunction".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.4.4 AsyncGeneratorFunction Instances

Every AsyncGeneratorFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse.

Each AsyncGeneratorFunction instance has the following own properties:

27.4.4.1 length

The value of the"length"property is anintegral Numberthat indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the language permits the function to be invoked with some other number of arguments. The behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its"length"property depends on the function.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.4.4.2 name

The specification for the"name"property of Function instances given in20.2.4.2also applies to AsyncGeneratorFunction instances.

27.4.4.3 prototype

Whenever an AsyncGeneratorFunction instance is created anotherordinary objectis also created and is the initial value of the async generator function's"prototype"property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator object when the generatorfunction objectis invoked using [[Call]].

This property has the attributes { [[Writable]]:true, [[Enumerable]]:false, [[Configurable]]:false}.

Note

Unlike function instances, the object that is the value of the an AsyncGeneratorFunction's"prototype"property does not have a"constructor"property whose value is the AsyncGeneratorFunction instance.

27.5 Generator Objects

A Generator object is an instance of a generator function and conforms to both the Iterator and Iterable interfaces.

Generator instances directly inherit properties from the object that is the initial value of the"prototype"property of the Generator function that created the instance. Generator instances indirectly inherit properties from the Generator Prototype intrinsic,%GeneratorFunction.prototype.prototype%.

27.5.1 Properties of the Generator Prototype Object

The Generator prototype object:

  • is %GeneratorFunction.prototype.prototype%.
  • is anordinary object.
  • is not a Generator instance and does not have a [[GeneratorState]] internal slot.
  • has a [[Prototype]] internal slot whose value is%IteratorPrototype%.
  • has properties that are indirectly inherited by all Generator instances.

27.5.1.1 Generator.prototype.constructor

The initial value of Generator.prototype.constructor is%GeneratorFunction.prototype%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.5.1.2 Generator.prototype.next ( value )

The next method performs the following steps:

  1. Let g be thethisvalue.
  2. Return ? GeneratorResume(g, value,empty).

27.5.1.3 Generator.prototype.return ( value )

The return method performs the following steps:

  1. Let g be thethisvalue.
  2. Let C beCompletion{ [[Type]]:return, [[Value]]: value, [[Target]]:empty}.
  3. Return ? GeneratorResumeAbrupt(g, C,empty).

27.5.1.4 Generator.prototype.throw ( exception )

The throw method performs the following steps:

  1. Let g be thethisvalue.
  2. Let C beThrowCompletion(exception).
  3. Return ? GeneratorResumeAbrupt(g, C,empty).

27.5.1.5 Generator.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Generator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.5.2 Properties of Generator Instances

Generator instances are initially created with the internal slots described inTable 76.

Table 76: Internal Slots of Generator Instances
Internal SlotDescription
[[GeneratorState]]The current execution state of the generator. The possible values are:undefined,suspendedStart,suspendedYield,executing, andcompleted.
[[GeneratorContext]]Theexecution contextthat is used when executing the code of this generator.
[[GeneratorBrand]]A brand used to distinguish different kinds of generators. The [[GeneratorBrand]] of generators declared by ECMAScript source text is alwaysempty.

27.5.3 Generator Abstract Operations

27.5.3.1 GeneratorStart ( generator, generatorBody )

The abstract operation GeneratorStart takes arguments generator and generatorBody (aParse Nodeor anAbstract Closurewith no parameters). It performs the following steps when called:

  1. Assert: The value of generator.[[GeneratorState]] isundefined.
  2. Let genContext be therunning execution context.
  3. Set the Generator component of genContext to generator.
  4. Set the code evaluation state of genContext such that when evaluation is resumed for thatexecution contextthe following steps will be performed:
    1. If generatorBody is aParse Node, then
      1. Let result be the result of evaluating generatorBody.
    2. Else,
      1. Assert: generatorBody is anAbstract Closurewith no parameters.
      2. Let result be generatorBody().
    3. Assert: If we return here, the generator either threw an exception or performed either an implicit or explicit return.
    4. Remove genContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
    5. Set generator.[[GeneratorState]] tocompleted.
    6. Once a generator enters thecompletedstate it never leaves it and its associatedexecution contextis never resumed. Any execution state associated with generator can be discarded at this point.
    7. If result.[[Type]] isnormal, let resultValue beundefined.
    8. Else if result.[[Type]] isreturn, let resultValue be result.[[Value]].
    9. Else,
      1. Assert: result.[[Type]] isthrow.
      2. ReturnCompletion(result).
    10. ReturnCreateIterResultObject(resultValue,true).
  5. Set generator.[[GeneratorContext]] to genContext.
  6. Set generator.[[GeneratorState]] tosuspendedStart.
  7. ReturnNormalCompletion(undefined).

27.5.3.2 GeneratorValidate ( generator, generatorBrand )

The abstract operation GeneratorValidate takes arguments generator and generatorBrand. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(generator, [[GeneratorState]]).
  2. Perform ? RequireInternalSlot(generator, [[GeneratorBrand]]).
  3. If generator.[[GeneratorBrand]] is not the same value as generatorBrand, throw aTypeErrorexception.
  4. Assert: generator also has a [[GeneratorContext]] internal slot.
  5. Let state be generator.[[GeneratorState]].
  6. If state isexecuting, throw aTypeErrorexception.
  7. Return state.

27.5.3.3 GeneratorResume ( generator, value, generatorBrand )

The abstract operation GeneratorResume takes arguments generator, value, and generatorBrand. It performs the following steps when called:

  1. Let state be ? GeneratorValidate(generator, generatorBrand).
  2. If state iscompleted, returnCreateIterResultObject(undefined,true).
  3. Assert: state is eithersuspendedStartorsuspendedYield.
  4. Let genContext be generator.[[GeneratorContext]].
  5. Let methodContext be therunning execution context.
  6. Suspend methodContext.
  7. Set generator.[[GeneratorState]] toexecuting.
  8. Push genContext onto theexecution context stack; genContext is now therunning execution context.
  9. Resume the suspended evaluation of genContext usingNormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
  10. Assert: When we return here, genContext has already been removed from theexecution context stackand methodContext is the currentlyrunning execution context.
  11. ReturnCompletion(result).

27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand )

The abstract operation GeneratorResumeAbrupt takes arguments generator, abruptCompletion (aCompletion Recordwhose [[Type]] isreturnorthrow), and generatorBrand. It performs the following steps when called:

  1. Let state be ? GeneratorValidate(generator, generatorBrand).
  2. If state issuspendedStart, then
    1. Set generator.[[GeneratorState]] tocompleted.
    2. Once a generator enters thecompletedstate it never leaves it and its associatedexecution contextis never resumed. Any execution state associated with generator can be discarded at this point.
    3. Set state tocompleted.
  3. If state iscompleted, then
    1. If abruptCompletion.[[Type]] isreturn, then
      1. ReturnCreateIterResultObject(abruptCompletion.[[Value]],true).
    2. ReturnCompletion(abruptCompletion).
  4. Assert: state issuspendedYield.
  5. Let genContext be generator.[[GeneratorContext]].
  6. Let methodContext be therunning execution context.
  7. Suspend methodContext.
  8. Set generator.[[GeneratorState]] toexecuting.
  9. Push genContext onto theexecution context stack; genContext is now therunning execution context.
  10. Resume the suspended evaluation of genContext using abruptCompletion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
  11. Assert: When we return here, genContext has already been removed from theexecution context stackand methodContext is the currentlyrunning execution context.
  12. ReturnCompletion(result).

27.5.3.5 GetGeneratorKind ( )

The abstract operation GetGeneratorKind takes no arguments. It performs the following steps when called:

  1. Let genContext be therunning execution context.
  2. If genContext does not have a Generator component, returnnon-generator.
  3. Let generator be the Generator component of genContext.
  4. If generator has an [[AsyncGeneratorState]] internal slot, returnasync.
  5. Else, returnsync.

27.5.3.6 GeneratorYield ( iterNextObj )

The abstract operation GeneratorYield takes argument iterNextObj. It performs the following steps when called:

  1. Assert: iterNextObj is an Object that implements the IteratorResult interface.
  2. Let genContext be therunning execution context.
  3. Assert: genContext is theexecution contextof a generator.
  4. Let generator be the value of the Generator component of genContext.
  5. Assert:GetGeneratorKind() issync.
  6. Set generator.[[GeneratorState]] tosuspendedYield.
  7. Remove genContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
  8. Set the code evaluation state of genContext such that when evaluation is resumed with aCompletionresumptionValue the following steps will be performed:
    1. Return resumptionValue.
    2. NOTE: This returns to the evaluation of theYieldExpressionthat originally called this abstract operation.
  9. ReturnNormalCompletion(iterNextObj).
  10. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.

27.5.3.7 Yield ( value )

The abstract operation Yield takes argument value (anECMAScript language value). It performs the following steps when called:

  1. Let generatorKind be ! GetGeneratorKind().
  2. If generatorKind isasync, return ? AsyncGeneratorYield(value).
  3. Otherwise, return ? GeneratorYield(!CreateIterResultObject(value,false)).

27.5.3.8 CreateIteratorFromClosure ( closure, generatorBrand, generatorPrototype )

The abstract operation CreateIteratorFromClosure takes arguments closure (anAbstract Closurewith no parameters), generatorBrand, and generatorPrototype (an Object). It performs the following steps when called:

  1. NOTE: closure can contain uses of theYieldshorthand to yield an IteratorResult object.
  2. Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
  3. Let generator be ! OrdinaryObjectCreate(generatorPrototype, internalSlotsList).
  4. Set generator.[[GeneratorBrand]] to generatorBrand.
  5. Set generator.[[GeneratorState]] toundefined.
  6. Let callerContext be therunning execution context.
  7. Let calleeContext be a newexecution context.
  8. Set the Function of calleeContext tonull.
  9. Set theRealmof calleeContext tothe current Realm Record.
  10. Set the ScriptOrModule of calleeContext to callerContext's ScriptOrModule.
  11. If callerContext is not already suspended, suspend callerContext.
  12. Push calleeContext onto theexecution context stack; calleeContext is now therunning execution context.
  13. Perform ! GeneratorStart(generator, closure).
  14. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
  15. Return generator.

27.6 AsyncGenerator Objects

An AsyncGenerator object is an instance of an async generator function and conforms to both the AsyncIterator and AsyncIterable interfaces.

AsyncGenerator instances directly inherit properties from the object that is the initial value of the"prototype"property of the AsyncGenerator function that created the instance. AsyncGenerator instances indirectly inherit properties from the AsyncGenerator Prototype intrinsic,%AsyncGeneratorFunction.prototype.prototype%.

27.6.1 Properties of the AsyncGenerator Prototype Object

The AsyncGenerator prototype object:

  • is %AsyncGeneratorFunction.prototype.prototype%.
  • is anordinary object.
  • is not an AsyncGenerator instance and does not have an [[AsyncGeneratorState]] internal slot.
  • has a [[Prototype]] internal slot whose value is%AsyncIteratorPrototype%.
  • has properties that are indirectly inherited by all AsyncGenerator instances.

27.6.1.1 AsyncGenerator.prototype.constructor

The initial value of AsyncGenerator.prototype.constructor is%AsyncGeneratorFunction.prototype%.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.6.1.2 AsyncGenerator.prototype.next ( value )

  1. Let generator be thethisvalue.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result beAsyncGeneratorValidate(generator,empty).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let state be generator.[[AsyncGeneratorState]].
  6. If state iscompleted, then
    1. Let iteratorResult be ! CreateIterResultObject(undefined,true).
    2. Perform ! Call(promiseCapability.[[Resolve]],undefined, « iteratorResult »).
    3. Return promiseCapability.[[Promise]].
  7. Let completion beNormalCompletion(value).
  8. Perform ! AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  9. If state is eithersuspendedStartorsuspendedYield, then
    1. Perform ! AsyncGeneratorResume(generator, completion).
  10. Else,
    1. Assert: state is eitherexecutingorawaiting-return.
  11. Return promiseCapability.[[Promise]].

27.6.1.3 AsyncGenerator.prototype.return ( value )

  1. Let generator be thethisvalue.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result beAsyncGeneratorValidate(generator,empty).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let completion beCompletion{ [[Type]]:return, [[Value]]: value, [[Target]]:empty}.
  6. Perform ! AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  7. Let state be generator.[[AsyncGeneratorState]].
  8. If state is eithersuspendedStartorcompleted, then
    1. Set generator.[[AsyncGeneratorState]] toawaiting-return.
    2. Perform ! AsyncGeneratorAwaitReturn(generator).
  9. Else if state issuspendedYield, then
    1. Perform ! AsyncGeneratorResume(generator, completion).
  10. Else,
    1. Assert: state is eitherexecutingorawaiting-return.
  11. Return promiseCapability.[[Promise]].

27.6.1.4 AsyncGenerator.prototype.throw ( exception )

  1. Let generator be thethisvalue.
  2. Let promiseCapability be ! NewPromiseCapability(%Promise%).
  3. Let result beAsyncGeneratorValidate(generator,empty).
  4. IfAbruptRejectPromise(result, promiseCapability).
  5. Let state be generator.[[AsyncGeneratorState]].
  6. If state issuspendedStart, then
    1. Set generator.[[AsyncGeneratorState]] tocompleted.
    2. Set state tocompleted.
  7. If state iscompleted, then
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « exception »).
    2. Return promiseCapability.[[Promise]].
  8. Let completion beThrowCompletion(exception).
  9. Perform ! AsyncGeneratorEnqueue(generator, completion, promiseCapability).
  10. If state issuspendedYield, then
    1. Perform ! AsyncGeneratorResume(generator, completion).
  11. Else,
    1. Assert: state is eitherexecutingorawaiting-return.
  12. Return promiseCapability.[[Promise]].

27.6.1.5 AsyncGenerator.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"AsyncGenerator".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.6.2 Properties of AsyncGenerator Instances

AsyncGenerator instances are initially created with the internal slots described below:

Table 77: Internal Slots of AsyncGenerator Instances
Internal SlotDescription
[[AsyncGeneratorState]]The current execution state of the async generator. The possible values are:undefined,suspendedStart,suspendedYield,executing,awaiting-return, andcompleted.
[[AsyncGeneratorContext]]Theexecution contextthat is used when executing the code of this async generator.
[[AsyncGeneratorQueue]]AListofAsyncGeneratorRequestrecords which represent requests to resume the async generator. Except during state transitions, it is nonempty if and only if [[AsyncGeneratorState]] is eitherexecutingorawaiting-return.
[[GeneratorBrand]]A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is alwaysempty.

27.6.3 AsyncGenerator Abstract Operations

27.6.3.1 AsyncGeneratorRequest Records

An AsyncGeneratorRequest is aRecordvalue used to store information about how an async generator should be resumed and contains capabilities for fulfilling or rejecting the corresponding promise.

They have the following fields:

Table 78: AsyncGeneratorRequestRecordFields
Field NameValueMeaning
[[Completion]]ACompletionrecordThe completion which should be used to resume the async generator.
[[Capability]]APromiseCapability RecordThe promise capabilities associated with this request.

27.6.3.2 AsyncGeneratorStart ( generator, generatorBody )

The abstract operation AsyncGeneratorStart takes arguments generator and generatorBody (aParse Nodeor anAbstract Closurewith no parameters). It performs the following steps when called:

  1. Assert: generator is an AsyncGenerator instance.
  2. Assert: generator.[[AsyncGeneratorState]] isundefined.
  3. Let genContext be therunning execution context.
  4. Set the Generator component of genContext to generator.
  5. Set the code evaluation state of genContext such that when evaluation is resumed for thatexecution contextthe following steps will be performed:
    1. If generatorBody is aParse Node, then
      1. Let result be the result of evaluating generatorBody.
    2. Else,
      1. Assert: generatorBody is anAbstract Closurewith no parameters.
      2. Let result be generatorBody().
    3. Assert: If we return here, the async generator either threw an exception or performed either an implicit or explicit return.
    4. Remove genContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
    5. Set generator.[[AsyncGeneratorState]] tocompleted.
    6. If result.[[Type]] isnormal, set result toNormalCompletion(undefined).
    7. If result.[[Type]] isreturn, set result toNormalCompletion(result.[[Value]]).
    8. Perform ! AsyncGeneratorCompleteStep(generator, result,true).
    9. Perform ! AsyncGeneratorDrainQueue(generator).
    10. Returnundefined.
  6. Set generator.[[AsyncGeneratorContext]] to genContext.
  7. Set generator.[[AsyncGeneratorState]] tosuspendedStart.
  8. Set generator.[[AsyncGeneratorQueue]] to a new emptyList.
  9. Returnundefined.

27.6.3.3 AsyncGeneratorValidate ( generator, generatorBrand )

The abstract operation AsyncGeneratorValidate takes arguments generator and generatorBrand. It performs the following steps when called:

  1. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorContext]]).
  2. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorState]]).
  3. Perform ? RequireInternalSlot(generator, [[AsyncGeneratorQueue]]).
  4. If generator.[[GeneratorBrand]] is not the same value as generatorBrand, throw aTypeErrorexception.

27.6.3.4 AsyncGeneratorEnqueue ( generator, completion, promiseCapability )

The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator), completion (aCompletion Record), and promiseCapability (aPromiseCapability Record). It performs the following steps when called:

  1. Let request beAsyncGeneratorRequest{ [[Completion]]: completion, [[Capability]]: promiseCapability }.
  2. Append request to the end of generator.[[AsyncGeneratorQueue]].

27.6.3.5 AsyncGeneratorCompleteStep ( generator, completion, done [ , realm ] )

The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an AsyncGenerator), completion (aCompletion Record), and done (a Boolean) and optional argument realm (aRealm Record). It performs the following steps when called:

  1. Let queue be generator.[[AsyncGeneratorQueue]].
  2. Assert: queue is not empty.
  3. Let next be the first element of queue.
  4. Remove the first element from queue.
  5. Let promiseCapability be next.[[Capability]].
  6. Let value be completion.[[Value]].
  7. If completion.[[Type]] isthrow, then
    1. Perform ! Call(promiseCapability.[[Reject]],undefined, « value »).
  8. Else,
    1. Assert: completion.[[Type]] isnormal.
    2. If realm is present, then
      1. Let oldRealm be therunning execution context'sRealm.
      2. Set therunning execution context'sRealmto realm.
      3. Let iteratorResult be ! CreateIterResultObject(value, done).
      4. Set therunning execution context'sRealmto oldRealm.
    3. Else,
      1. Let iteratorResult be ! CreateIterResultObject(value, done).
    4. Perform ! Call(promiseCapability.[[Resolve]],undefined, « iteratorResult »).

27.6.3.6 AsyncGeneratorResume ( generator, completion )

The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and completion (aCompletion Record). It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorState]] is eithersuspendedStartorsuspendedYield.
  2. Let genContext be generator.[[AsyncGeneratorContext]].
  3. Let callerContext be therunning execution context.
  4. Suspend callerContext.
  5. Set generator.[[AsyncGeneratorState]] toexecuting.
  6. Push genContext onto theexecution context stack; genContext is now therunning execution context.
  7. Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the completion record returned by the resumed computation.
  8. Assert: result is never anabrupt completion.
  9. Assert: When we return here, genContext has already been removed from theexecution context stackand callerContext is the currentlyrunning execution context.

27.6.3.7 AsyncGeneratorUnwrapYieldResumption ( resumptionValue )

The abstract operation AsyncGeneratorUnwrapYieldResumption takes argument resumptionValue (aCompletion Record). It performs the following steps when called:

  1. If resumptionValue.[[Type]] is notreturn, returnCompletion(resumptionValue).
  2. Let awaited beAwait(resumptionValue.[[Value]]).
  3. If awaited.[[Type]] isthrow, returnCompletion(awaited).
  4. Assert: awaited.[[Type]] isnormal.
  5. ReturnCompletion{ [[Type]]:return, [[Value]]: awaited.[[Value]], [[Target]]:empty}.

27.6.3.8 AsyncGeneratorYield ( value )

The abstract operation AsyncGeneratorYield takes argument value. It performs the following steps when called:

  1. Let genContext be therunning execution context.
  2. Assert: genContext is theexecution contextof a generator.
  3. Let generator be the value of the Generator component of genContext.
  4. Assert:GetGeneratorKind() isasync.
  5. Set value to ? Await(value).
  6. Let completion beNormalCompletion(value).
  7. Assert: Theexecution context stackhas at least two elements.
  8. Let previousContext be the second to top element of theexecution context stack.
  9. Let previousRealm be previousContext'sRealm.
  10. Perform ! AsyncGeneratorCompleteStep(generator, completion,false, previousRealm).
  11. Let queue be generator.[[AsyncGeneratorQueue]].
  12. If queue is not empty, then
    1. NOTE: Execution continues without suspending the generator.
    2. Let toYield be the first element of queue.
    3. Let resumptionValue be toYield.[[Completion]].
    4. ReturnAsyncGeneratorUnwrapYieldResumption(resumptionValue).
  13. Else,
    1. Set generator.[[AsyncGeneratorState]] tosuspendedYield.
    2. Remove genContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
    3. Set the code evaluation state of genContext such that when evaluation is resumed with aCompletionresumptionValue the following steps will be performed:
      1. ReturnAsyncGeneratorUnwrapYieldResumption(resumptionValue).
      2. NOTE: When the above step returns, it returns to the evaluation of theYieldExpressionproduction that originally called this abstract operation.
    4. Returnundefined.
    5. NOTE: This returns to the evaluation of the operation that had most previously resumed evaluation of genContext.

27.6.3.9 AsyncGeneratorAwaitReturn ( generator )

The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an AsyncGenerator). It performs the following steps when called:

  1. Let queue be generator.[[AsyncGeneratorQueue]].
  2. Assert: queue is not empty.
  3. Let next be the first element of queue.
  4. Let completion be next.[[Completion]].
  5. Assert: completion.[[Type]] isreturn.
  6. Let promise be ? PromiseResolve(%Promise%, completion.[[Value]]).
  7. Let fulfilledClosure be a newAbstract Closurewith parameters (value) that captures generator and performs the following steps when called:
    1. Set generator.[[AsyncGeneratorState]] tocompleted.
    2. Let result beNormalCompletion(value).
    3. Perform ! AsyncGeneratorCompleteStep(generator, result,true).
    4. Perform ! AsyncGeneratorDrainQueue(generator).
    5. Returnundefined.
  8. Let onFulfilled be ! CreateBuiltinFunction(fulfilledClosure, 1,"", « »).
  9. Let rejectedClosure be a newAbstract Closurewith parameters (reason) that captures generator and performs the following steps when called:
    1. Set generator.[[AsyncGeneratorState]] tocompleted.
    2. Let result beThrowCompletion(reason).
    3. Perform ! AsyncGeneratorCompleteStep(generator, result,true).
    4. Perform ! AsyncGeneratorDrainQueue(generator).
    5. Returnundefined.
  10. Let onRejected be ! CreateBuiltinFunction(rejectedClosure, 1,"", « »).
  11. Perform ! PerformPromiseThen(promise, onFulfilled, onRejected).

27.6.3.10 AsyncGeneratorDrainQueue ( generator )

The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator). It drains the generator's AsyncGeneratorQueue until it encounters anAsyncGeneratorRequestwhich holds a completion whose type isreturn. It performs the following steps when called:

  1. Assert: generator.[[AsyncGeneratorState]] iscompleted.
  2. Let queue be generator.[[AsyncGeneratorQueue]].
  3. If queue is empty, return.
  4. Let done befalse.
  5. Repeat, while done isfalse,
    1. Let next be the first element of queue.
    2. Let completion be next.[[Completion]].
    3. If completion.[[Type]] isreturn, then
      1. Set generator.[[AsyncGeneratorState]] toawaiting-return.
      2. Perform ! AsyncGeneratorAwaitReturn(generator).
      3. Set done totrue.
    4. Else,
      1. If completion.[[Type]] isnormal, then
        1. Set completion toNormalCompletion(undefined).
      2. Perform ! AsyncGeneratorCompleteStep(generator, completion,true).
      3. If queue is empty, set done totrue.

27.6.3.11 CreateAsyncIteratorFromClosure ( closure, generatorBrand, generatorPrototype )

The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (anAbstract Closurewith no parameters), generatorBrand, and generatorPrototype (an Object). It performs the following steps when called:

  1. NOTE: closure can contain uses of theAwaitshorthand and uses of theYieldshorthand to yield an IteratorResult object.
  2. Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]], [[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
  3. Let generator be ! OrdinaryObjectCreate(generatorPrototype, internalSlotsList).
  4. Set generator.[[GeneratorBrand]] to generatorBrand.
  5. Set generator.[[AsyncGeneratorState]] toundefined.
  6. Let callerContext be therunning execution context.
  7. Let calleeContext be a newexecution context.
  8. Set the Function of calleeContext tonull.
  9. Set theRealmof calleeContext tothe current Realm Record.
  10. Set the ScriptOrModule of calleeContext to callerContext's ScriptOrModule.
  11. If callerContext is not already suspended, suspend callerContext.
  12. Push calleeContext onto theexecution context stack; calleeContext is now therunning execution context.
  13. Perform ! AsyncGeneratorStart(generator, closure).
  14. Remove calleeContext from theexecution context stackand restore callerContext as therunning execution context.
  15. Return generator.

27.7 AsyncFunction Objects

AsyncFunction objects are functions that are usually created by evaluatingAsyncFunctionDeclarations,AsyncFunctionExpressions,AsyncMethods, andAsyncArrowFunctions. They may also be created by calling the%AsyncFunction%intrinsic.

27.7.1 The AsyncFunction Constructor

The AsyncFunctionconstructor:

  • is %AsyncFunction%.
  • is a subclass of Function.
  • creates and initializes a new AsyncFunction object when called as a function rather than as aconstructor. Thus the function call AsyncFunction(…) is equivalent to the object creation expression new AsyncFunction(…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified AsyncFunction behaviour must include a super call to the AsyncFunctionconstructorto create and initialize a subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript syntactic forms for defining async function objects create direct instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.

27.7.1.1 AsyncFunction ( p1, p2, … , pn, body )

The last argument specifies the body (executable code) of an async function. Any preceding arguments specify formal parameters.

When the AsyncFunction function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is, there are no p arguments, and where body might also not be provided), the following steps are taken:

  1. Let C be theactive function object.
  2. Let args be the argumentsList that was passed to this function by [[Call]] or [[Construct]].
  3. ReturnCreateDynamicFunction(C, NewTarget,async, args).
Note
See NOTE for20.2.1.1.

27.7.2 Properties of the AsyncFunction Constructor

The AsyncFunctionconstructor:

  • is a standard built-infunction objectthat inherits from the Functionconstructor.
  • has a [[Prototype]] internal slot whose value is%Function%.
  • has a"name"property whose value is"AsyncFunction".
  • has the following properties:

27.7.2.1 AsyncFunction.length

This is adata propertywith a value of 1. This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.7.2.2 AsyncFunction.prototype

The initial value of AsyncFunction.prototype is theAsyncFunction prototype object.

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

27.7.3 Properties of the AsyncFunction Prototype Object

The AsyncFunction prototype object:

27.7.3.1 AsyncFunction.prototype.constructor

The initial value of AsyncFunction.prototype.constructor is%AsyncFunction%

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.7.3.2 AsyncFunction.prototype [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"AsyncFunction".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

27.7.4 AsyncFunction Instances

Every AsyncFunction instance is an ECMAScriptfunction objectand has the internal slots listed inTable 33. The value of the [[IsClassConstructor]] internal slot for all such instances isfalse. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype property as they are not constructible.

Each AsyncFunction instance has the following own properties:

27.7.4.1 length

The specification for the"length"property of Function instances given in20.2.4.1also applies to AsyncFunction instances.

27.7.4.2 name

The specification for the"name"property of Function instances given in20.2.4.2also applies to AsyncFunction instances.

27.7.5 Async Functions Abstract Operations

27.7.5.1 AsyncFunctionStart ( promiseCapability, asyncFunctionBody )

The abstract operation AsyncFunctionStart takes arguments promiseCapability (aPromiseCapability Record) and asyncFunctionBody. It performs the following steps when called:

  1. Let runningContext be therunning execution context.
  2. Let asyncContext be a copy of runningContext.
  3. NOTE: Copying the execution state is required for the step below to resume its execution. It is ill-defined to resume a currently executing context.
  4. Set the code evaluation state of asyncContext such that when evaluation is resumed for thatexecution contextthe following steps will be performed:
    1. Let result be the result of evaluating asyncFunctionBody.
    2. Assert: If we return here, the async function either threw an exception or performed an implicit or explicit return; all awaiting is done.
    3. Remove asyncContext from theexecution context stackand restore theexecution contextthat is at the top of theexecution context stackas therunning execution context.
    4. If result.[[Type]] isnormal, then
      1. Perform ! Call(promiseCapability.[[Resolve]],undefined, «undefined»).
    5. Else if result.[[Type]] isreturn, then
      1. Perform ! Call(promiseCapability.[[Resolve]],undefined, « result.[[Value]] »).
    6. Else,
      1. Assert: result.[[Type]] isthrow.
      2. Perform ! Call(promiseCapability.[[Reject]],undefined, « result.[[Value]] »).
    7. Return.
  5. Push asyncContext onto theexecution context stack; asyncContext is now therunning execution context.
  6. Resume the suspended evaluation of asyncContext. Let result be the value returned by the resumed computation.
  7. Assert: When we return here, asyncContext has already been removed from theexecution context stackand runningContext is the currentlyrunning execution context.
  8. Assert: result is anormal completionwith a value ofundefined. The possible sources of completion values areAwaitor, if the async function doesn't await anything, step4.gabove.
  9. Return.

28 Reflection

28.1 The Reflect Object

The Reflect object:

  • is %Reflect%.
  • is the initial value of the"Reflect"property of theglobal object.
  • is anordinary object.
  • has a [[Prototype]] internal slot whose value is%Object.prototype%.
  • is not afunction object.
  • does not have a [[Construct]] internal method; it cannot be used as aconstructorwith the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

28.1.1 Reflect.apply ( target, thisArgument, argumentsList )

When the apply function is called with arguments target, thisArgument, and argumentsList, the following steps are taken:

  1. IfIsCallable(target) isfalse, throw aTypeErrorexception.
  2. Let args be ? CreateListFromArrayLike(argumentsList).
  3. PerformPrepareForTailCall().
  4. Return ? Call(target, thisArgument, args).

28.1.2 Reflect.construct ( target, argumentsList [ , newTarget ] )

When the construct function is called with arguments target, argumentsList, and newTarget, the following steps are taken:

  1. IfIsConstructor(target) isfalse, throw aTypeErrorexception.
  2. If newTarget is not present, set newTarget to target.
  3. Else ifIsConstructor(newTarget) isfalse, throw aTypeErrorexception.
  4. Let args be ? CreateListFromArrayLike(argumentsList).
  5. Return ? Construct(target, args, newTarget).

28.1.3 Reflect.defineProperty ( target, propertyKey, attributes )

When the defineProperty function is called with arguments target, propertyKey, and attributes, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Let desc be ? ToPropertyDescriptor(attributes).
  4. Return ? target.[[DefineOwnProperty]](key, desc).

28.1.4 Reflect.deleteProperty ( target, propertyKey )

When the deleteProperty function is called with arguments target and propertyKey, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Return ? target.[[Delete]](key).

28.1.5 Reflect.get ( target, propertyKey [ , receiver ] )

When the get function is called with arguments target, propertyKey, and receiver, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Get]](key, receiver).

28.1.6 Reflect.getOwnPropertyDescriptor ( target, propertyKey )

When the getOwnPropertyDescriptor function is called with arguments target and propertyKey, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Let desc be ? target.[[GetOwnProperty]](key).
  4. ReturnFromPropertyDescriptor(desc).

28.1.7 Reflect.getPrototypeOf ( target )

When the getPrototypeOf function is called with argument target, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Return ? target.[[GetPrototypeOf]]().

28.1.8 Reflect.has ( target, propertyKey )

When the has function is called with arguments target and propertyKey, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. Return ? target.[[HasProperty]](key).

28.1.9 Reflect.isExtensible ( target )

When the isExtensible function is called with argument target, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Return ? target.[[IsExtensible]]().

28.1.10 Reflect.ownKeys ( target )

When the ownKeys function is called with argument target, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let keys be ? target.[[OwnPropertyKeys]]().
  3. ReturnCreateArrayFromList(keys).

28.1.11 Reflect.preventExtensions ( target )

When the preventExtensions function is called with argument target, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Return ? target.[[PreventExtensions]]().

28.1.12 Reflect.set ( target, propertyKey, V [ , receiver ] )

When the set function is called with arguments target, V, propertyKey, and receiver, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. Let key be ? ToPropertyKey(propertyKey).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Set]](key, V, receiver).

28.1.13 Reflect.setPrototypeOf ( target, proto )

When the setPrototypeOf function is called with arguments target and proto, the following steps are taken:

  1. IfType(target) is not Object, throw aTypeErrorexception.
  2. IfType(proto) is not Object and proto is notnull, throw aTypeErrorexception.
  3. Return ? target.[[SetPrototypeOf]](proto).

28.1.14 Reflect [ @@toStringTag ]

The initial value of the@@toStringTagproperty is the String value"Reflect".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:true}.

28.2 Proxy Objects

28.2.1 The Proxy Constructor

The Proxyconstructor:

  • is %Proxy%.
  • is the initial value of the"Proxy"property of theglobal object.
  • creates and initializes a newProxy exotic objectwhen called as aconstructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.

28.2.1.1 Proxy ( target, handler )

When Proxy is called with arguments target and handler, it performs the following steps:

  1. If NewTarget isundefined, throw aTypeErrorexception.
  2. Return ? ProxyCreate(target, handler).

28.2.2 Properties of the Proxy Constructor

The Proxyconstructor:

  • has a [[Prototype]] internal slot whose value is%Function.prototype%.
  • does not have a"prototype"property because Proxy exotic objects do not have a [[Prototype]] internal slot that requires initialization.
  • has the following properties:

28.2.2.1 Proxy.revocable ( target, handler )

The Proxy.revocable function is used to create a revocable Proxy object. When Proxy.revocable is called with arguments target and handler, the following steps are taken:

  1. Let p be ? ProxyCreate(target, handler).
  2. Let revokerClosure be a newAbstract Closurewith no parameters that captures nothing and performs the following steps when called:
    1. Let F be theactive function object.
    2. Let p be F.[[RevocableProxy]].
    3. If p isnull, returnundefined.
    4. Set F.[[RevocableProxy]] tonull.
    5. Assert: p is a Proxy object.
    6. Set p.[[ProxyTarget]] tonull.
    7. Set p.[[ProxyHandler]] tonull.
    8. Returnundefined.
  3. Let revoker be ! CreateBuiltinFunction(revokerClosure, 0,"", « [[RevocableProxy]] »).
  4. Set revoker.[[RevocableProxy]] to p.
  5. Let result be ! OrdinaryObjectCreate(%Object.prototype%).
  6. Perform ! CreateDataPropertyOrThrow(result,"proxy", p).
  7. Perform ! CreateDataPropertyOrThrow(result,"revoke", revoker).
  8. Return result.

28.3 Module Namespace Objects

A Module Namespace Object is amodule namespace exotic objectthat provides runtime property-based access to a module's exported bindings. There is noconstructorfunction for Module Namespace Objects. Instead, such an object is created for each module that is imported by anImportDeclarationthat contains aNameSpaceImport.

In addition to the properties specified in10.4.6each Module Namespace Object has the following own property:

28.3.1 @@toStringTag

The initial value of the@@toStringTagproperty is the String value"Module".

This property has the attributes { [[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}.

29 Memory Model

The memory consistency model, or memory model, specifies the possible orderings ofShared Data Blockevents, arising via accessing TypedArray instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from eachagent. When the program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible values read from data races. To wit, there is no undefined behaviour.

The memory model is defined as relational constraints on events introduced byabstract operationson SharedArrayBuffer or by methods on the Atomics object during an evaluation.

Note

This section provides an axiomatic model on events introduced by theabstract operationson SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically, unlike the rest of this specification. The nondeterministic introduction of events byabstract operationsis the interface between the operational semantics of ECMAScript evaluation and the axiomatic semantics of the memory model. The semantics of these events is defined by considering graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular event graph is allowed or disallowed.

29.1 Memory Model Fundamentals

Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events agreed upon by all agents in anagent cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all agents, i.e., unordered.

Note 1

No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire, are supported.

A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemoryRecord.

Table 79:ReadSharedMemoryEvent Fields
Field NameValueMeaning
[[Order]]SeqCst|UnorderedThe weakest ordering guaranteed by thememory modelfor the event.
[[NoTear]]A BooleanWhether this event is allowed to read from multiple write events on equal range as this event.
[[Block]]AShared Data BlockThe block the event operates on.
[[ByteIndex]]A non-negativeintegerThe byte address of the read in [[Block]].
[[ElementSize]]A non-negativeintegerThe size of the read.
Table 80:WriteSharedMemoryEvent Fields
Field NameValueMeaning
[[Order]]SeqCst|Unordered|InitThe weakest ordering guaranteed by thememory modelfor the event.
[[NoTear]]A BooleanWhether this event is allowed to be read from multiple read events with equal range as this event.
[[Block]]AShared Data BlockThe block the event operates on.
[[ByteIndex]]A non-negativeintegerThe byte address of the write in [[Block]].
[[ElementSize]]A non-negativeintegerThe size of the write.
[[Payload]]AListTheListof byte values to be read by other events.
Table 81:ReadModifyWriteSharedMemoryEvent Fields
Field NameValueMeaning
[[Order]]SeqCstRead-modify-write events are always sequentially consistent.
[[NoTear]]trueRead-modify-write events cannot tear.
[[Block]]AShared Data BlockThe block the event operates on.
[[ByteIndex]]A non-negativeintegerThe byte address of the read-modify-write in [[Block]].
[[ElementSize]]A non-negativeintegerThe size of the read-modify-write.
[[Payload]]AListTheListof byte values to be passed to [[ModifyOp]].
[[ModifyOp]]Aread-modify-write modification functionAn abstract closure that returns a modifiedListof byte values from a readListof byte values and [[Payload]].

These events are introduced byabstract operationsor by methods on the Atomics object.

Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to directly constrain the permitted orderings of other events.

In addition toShared Data Blockand Synchronize events, there arehost-specific events.

Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.

Note 2

Examples ofhost-specific synchronizing events that should be accounted for are: sending a SharedArrayBuffer from oneagentto another (e.g., by postMessage in a browser), starting and stopping agents, and communicating within theagent clustervia channels other than shared memory. It is assumed those events are appended toagent-orderduring evaluation like the other SharedArrayBuffer events.

Events are ordered within candidate executions by the relations defined below.

29.2 Agent Events Records

An Agent Events Record is aRecordwith the following fields.

Table 82:Agent Events RecordFields
Field NameValueMeaning
[[AgentSignifier]]A value that admits equality testingTheagentwhose evaluation resulted in this ordering.
[[EventList]]AListof eventsEvents are appended to the list during evaluation.
[[AgentSynchronizesWith]]AListof pairs ofSynchronizeeventsSynchronizerelationships introduced by the operational semantics.

29.3 Chosen Value Records

A Chosen Value Record is aRecordwith the following fields.

Table 83:Chosen Value RecordFields
Field NameValueMeaning
[[Event]]AShared Data Block eventTheReadSharedMemoryorReadModifyWriteSharedMemoryevent that was introduced for this chosen value.
[[ChosenValue]]AListof byte valuesThe bytes that were nondeterministically chosen during evaluation.

29.4 Candidate Executions

A candidate execution of the evaluation of anagent clusteris aRecordwith the following fields.

Table 84: Candidate ExecutionRecordFields
Field NameValueMeaning
[[EventsRecords]]AListofAgentEvents Records.Maps anagentto Lists of events appended during the evaluation.
[[ChosenValues]]AListof Chosen Value Records.MapsReadSharedMemoryorReadModifyWriteSharedMemoryevents to theListof byte values chosen during the evaluation.
[[AgentOrder]]Anagent-orderRelation.Defined below.
[[ReadsBytesFrom]]Areads-bytes-frommathematical function.Defined below.
[[ReadsFrom]]Areads-fromRelation.Defined below.
[[HostSynchronizesWith]]Ahost-synchronizes-withRelation.Defined below.
[[SynchronizesWith]]Asynchronizes-withRelation.Defined below.
[[HappensBefore]]Ahappens-beforeRelation.Defined below.

An empty candidate execution is a candidate executionRecordwhose fields are empty Lists and Relations.

29.5 Abstract Operations for the Memory Model

29.5.1 EventSet ( execution )

The abstract operation EventSet takes argument execution (acandidate execution). It performs the following steps when called:

  1. Let events be an empty Set.
  2. For eachAgent Events Recordaer of execution.[[EventsRecords]], do
    1. For each event E of aer.[[EventList]], do
      1. Add E to events.
  3. Return events.

29.5.2 SharedDataBlockEventSet ( execution )

The abstract operation SharedDataBlockEventSet takes argument execution (acandidate execution). It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E ofEventSet(execution), do
    1. If E is aReadSharedMemory,WriteSharedMemory, orReadModifyWriteSharedMemoryevent, add E to events.
  3. Return events.

29.5.3 HostEventSet ( execution )

The abstract operation HostEventSet takes argument execution (acandidate execution). It performs the following steps when called:

  1. Let events be an empty Set.
  2. For each event E ofEventSet(execution), do
    1. If E is not inSharedDataBlockEventSet(execution), add E to events.
  3. Return events.

29.5.4 ComposeWriteEventBytes ( execution, byteIndex, Ws )

The abstract operation ComposeWriteEventBytes takes arguments execution (acandidate execution), byteIndex (a non-negativeinteger), and Ws (aListofWriteSharedMemoryorReadModifyWriteSharedMemoryevents). It performs the following steps when called:

  1. Let byteLocation be byteIndex.
  2. Let bytesRead be a new emptyList.
  3. For each element W of Ws, do
    1. Assert: W has byteLocation in its range.
    2. Let payloadIndex be byteLocation - W.[[ByteIndex]].
    3. If W is aWriteSharedMemoryevent, then
      1. Let byte be W.[[Payload]][payloadIndex].
    4. Else,
      1. Assert: W is aReadModifyWriteSharedMemoryevent.
      2. Let bytes beValueOfReadEvent(execution, W).
      3. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
      4. Let byte be bytesModified[payloadIndex].
    5. Append byte to bytesRead.
    6. Set byteLocation to byteLocation + 1.
  4. Return bytesRead.
Note 1

The read-modify-write modification [[ModifyOp]] is given by the function properties on the Atomics object that introduceReadModifyWriteSharedMemoryevents.

Note 2

This abstract operation composes aListof write events into aListof byte values. It is used in the event semantics ofReadSharedMemoryandReadModifyWriteSharedMemoryevents.

29.5.5 ValueOfReadEvent ( execution, R )

The abstract operation ValueOfReadEvent takes arguments execution (acandidate execution) and R (aReadSharedMemoryorReadModifyWriteSharedMemoryevent). It performs the following steps when called:

  1. Assert: R is aReadSharedMemoryorReadModifyWriteSharedMemoryevent.
  2. Let Ws be execution.[[ReadsBytesFrom]](R).
  3. Assert: Ws is aListofWriteSharedMemoryorReadModifyWriteSharedMemoryevents with length equal to R.[[ElementSize]].
  4. ReturnComposeWriteEventBytes(execution, R.[[ByteIndex]], Ws).

29.6 Relations of Candidate Executions

29.6.1 agent-order

For acandidate executionexecution, execution.[[AgentOrder]] is aRelationon events that satisfies the following.

  • For each pair (E, D) inEventSet(execution), (E, D) is in execution.[[AgentOrder]] if there is someAgent Events Recordaer in execution.[[EventsRecords]] such that E and D are in aer.[[EventList]] and E is before D inListorder of aer.[[EventList]].
Note

Eachagentintroduces events in a per-agentstrict total orderduring the evaluation. This is the union of those strict total orders.

29.6.2 reads-bytes-from

For acandidate executionexecution, execution.[[ReadsBytesFrom]] is a mathematical function mapping events inSharedDataBlockEventSet(execution) to Lists of events inSharedDataBlockEventSet(execution) that satisfies the following conditions.

29.6.3 reads-from

For acandidate executionexecution, execution.[[ReadsFrom]] is the leastRelationon events that satisfies the following.

  • For each pair (R, W) inSharedDataBlockEventSet(execution), (R, W) is in execution.[[ReadsFrom]] if W is in execution.[[ReadsBytesFrom]](R).

29.6.4 host-synchronizes-with

For acandidate executionexecution, execution.[[HostSynchronizesWith]] is ahost-providedstrict partial orderonhost-specific events that satisfies at least the following.

  • If (E, D) is in execution.[[HostSynchronizesWith]], E and D are inHostEventSet(execution).
  • There is no cycle in the union of execution.[[HostSynchronizesWith]] and execution.[[AgentOrder]].
Note 1

For twohost-specific events E and D, E host-synchronizes-with D implies Ehappens-beforeD.

Note 2

The host-synchronizes-with relation allows thehostto provide additional synchronization mechanisms, such as postMessage between HTML workers.

29.6.5 synchronizes-with

For acandidate executionexecution, execution.[[SynchronizesWith]] is the leastRelationon events that satisfies the following.

  • For each pair (R, W) in execution.[[ReadsFrom]], (W, R) is in execution.[[SynchronizesWith]] if R.[[Order]] isSeqCst, W.[[Order]] isSeqCst, and R and W have equal ranges.
  • For each element eventsRecord of execution.[[EventsRecords]], the following is true.
    • For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], (S, Sw) is in execution.[[SynchronizesWith]].
  • For each pair (E, D) in execution.[[HostSynchronizesWith]], (E, D) is in execution.[[SynchronizesWith]].
Note 1

Owing to convention, write events synchronizes-with read events, instead of read events synchronizes-with write events.

Note 2

Initevents do not participate in synchronizes-with, and are instead constrained directly byhappens-before.

Note 3

Not allSeqCstevents related byreads-fromare related by synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.

Note 4

ForShared Data Blockevents R and W such that W synchronizes-with R, R mayreads-fromother writes than W.

29.6.6 happens-before

For acandidate executionexecution, execution.[[HappensBefore]] is the leastRelationon events that satisfies the following.

  • For each pair (E, D) in execution.[[AgentOrder]], (E, D) is in execution.[[HappensBefore]].
  • For each pair (E, D) in execution.[[SynchronizesWith]], (E, D) is in execution.[[HappensBefore]].
  • For each pair (E, D) inSharedDataBlockEventSet(execution), (E, D) is in execution.[[HappensBefore]] if E.[[Order]] isInitand E and D have overlapping ranges.
  • For each pair (E, D) inEventSet(execution), (E, D) is in execution.[[HappensBefore]] if there is an event F such that the pairs (E, F) and (F, D) are in execution.[[HappensBefore]].
Note

Because happens-before is a superset ofagent-order, candidate executions are consistent with the single-thread evaluation semantics of ECMAScript.

29.7 Properties of Valid Executions

29.7.1 Valid Chosen Reads

Acandidate executionexecution has valid chosen reads if the following algorithm returnstrue.

  1. For eachReadSharedMemoryorReadModifyWriteSharedMemoryevent R ofSharedDataBlockEventSet(execution), do
    1. Let chosenValueRecord be the element of execution.[[ChosenValues]] whose [[Event]] field is R.
    2. Let chosenValue be chosenValueRecord.[[ChosenValue]].
    3. Let readValue beValueOfReadEvent(execution, R).
    4. Let chosenLen be the number of elements of chosenValue.
    5. Let readLen be the number of elements of readValue.
    6. If chosenLenreadLen, then
      1. Returnfalse.
    7. If chosenValue[i] ≠ readValue[i] for anyintegervalue i in the range 0 through chosenLen, exclusive, then
      1. Returnfalse.
  2. Returntrue.

29.7.2 Coherent Reads

Acandidate executionexecution has coherent reads if the following algorithm returnstrue.

  1. For eachReadSharedMemoryorReadModifyWriteSharedMemoryevent R ofSharedDataBlockEventSet(execution), do
    1. Let Ws be execution.[[ReadsBytesFrom]](R).
    2. Let byteLocation be R.[[ByteIndex]].
    3. For each element W of Ws, do
      1. If (R, W) is in execution.[[HappensBefore]], then
        1. Returnfalse.
      2. If there is aWriteSharedMemoryorReadModifyWriteSharedMemoryevent V that has byteLocation in its range such that the pairs (W, V) and (V, R) are in execution.[[HappensBefore]], then
        1. Returnfalse.
      3. Set byteLocation to byteLocation + 1.
  2. Returntrue.

29.7.3 Tear Free Reads

Acandidate executionexecution has tear free reads if the following algorithm returnstrue.

  1. For eachReadSharedMemoryorReadModifyWriteSharedMemoryevent R ofSharedDataBlockEventSet(execution), do
    1. If R.[[NoTear]] istrue, then
      1. Assert: The remainder of dividing R.[[ByteIndex]] by R.[[ElementSize]] is 0.
      2. For each event W such that (R, W) is in execution.[[ReadsFrom]] and W.[[NoTear]] istrue, do
        1. If R and W have equal ranges, and there is an event V such that V and W have equal ranges, V.[[NoTear]] istrue, W is not V, and (R, V) is in execution.[[ReadsFrom]], then
          1. Returnfalse.
  2. Returntrue.
Note

An event's [[NoTear]] field istruewhen that event was introduced via accessing anintegerTypedArray, andfalsewhen introduced via accessing a floating point TypedArray or DataView.

Intuitively, this requirement says when a memory range is accessed in an aligned fashion via anintegerTypedArray, a single write event on that range must "win" when in a data race with other write events with equal ranges. More precisely, this requirement says an aligned read event cannot read a value composed of bytes from multiple, different write events all with equal ranges. It is possible, however, for an aligned read event to read from multiple write events with overlapping ranges.

29.7.4 Sequentially Consistent Atomics

For acandidate executionexecution, memory-order is astrict total orderof all events inEventSet(execution) that satisfies the following.

  • For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.
  • For each pair (R, W) in execution.[[ReadsFrom]], there is noWriteSharedMemoryorReadModifyWriteSharedMemoryevent V inSharedDataBlockEventSet(execution) such that V.[[Order]] isSeqCst, the pairs (W, V) and (V, R) are in memory-order, and any of the following conditions are true.

    • The pair (W, R) is in execution.[[SynchronizesWith]], and V and R have equal ranges.
    • The pairs (W, R) and (V, R) are in execution.[[HappensBefore]], W.[[Order]] isSeqCst, and W and V have equal ranges.
    • The pairs (W, R) and (W, V) are in execution.[[HappensBefore]], R.[[Order]] isSeqCst, and V and R have equal ranges.
    Note 1

    This clause additionally constrainsSeqCstevents on equal ranges.

  • For eachWriteSharedMemoryorReadModifyWriteSharedMemoryevent W inSharedDataBlockEventSet(execution), if W.[[Order]] isSeqCst, then it is not the case that there is an infinite number ofReadSharedMemoryorReadModifyWriteSharedMemoryevents inSharedDataBlockEventSet(execution) with equal range that is memory-order before W.

    Note 2

    This clause together with the forward progress guarantee on agents ensure the liveness condition thatSeqCstwrites become visible toSeqCstreads with equal range in finite time.

Acandidate executionhas sequentially consistent atomics if a memory-order exists.

Note 3

While memory-order includes all events inEventSet(execution), those that are not constrained byhappens-beforeorsynchronizes-withare allowed to occur anywhere in the order.

29.7.5 Valid Executions

Acandidate executionexecution is a valid execution (or simply an execution) if all of the following are true.

  • Thehostprovides ahost-synchronizes-withRelationfor execution.[[HostSynchronizesWith]].
  • execution.[[HappensBefore]] is astrict partial order.
  • execution has valid chosen reads.
  • execution has coherent reads.
  • execution has tear free reads.
  • execution has sequentially consistent atomics.

All programs have at least one valid execution.

29.8 Races

For an execution execution, two events E and D inSharedDataBlockEventSet(execution) are in a race if the following algorithm returnstrue.

  1. If E is not D, then
    1. If the pairs (E, D) and (D, E) are not in execution.[[HappensBefore]], then
      1. If E and D are bothWriteSharedMemoryorReadModifyWriteSharedMemoryevents and E and D do not have disjoint ranges, then
        1. Returntrue.
      2. If either (E, D) or (D, E) is in execution.[[ReadsFrom]], then
        1. Returntrue.
  2. Returnfalse.

29.9 Data Races

For an execution execution, two events E and D inSharedDataBlockEventSet(execution) are in a data race if the following algorithm returnstrue.

  1. If E and D are in a race in execution, then
    1. If E.[[Order]] is notSeqCstor D.[[Order]] is notSeqCst, then
      1. Returntrue.
    2. If E and D have overlapping ranges, then
      1. Returntrue.
  2. Returnfalse.

29.10 Data Race Freedom

An execution execution is data race free if there are no two events inSharedDataBlockEventSet(execution) that are in a data race.

A program is data race free if all its executions are data race free.

Thememory modelguarantees sequential consistency of all events for data race free programs.

29.11 Shared Memory Guidelines

Note 1

The following are guidelines for ECMAScript programmers working with shared memory.

We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be concurrent non-atomic operations on the same memory location. Data race free programs have interleaving semantics where each step in the evaluation semantics of eachagentare interleaved with each other. For data race free programs, it is not necessary to understand the details of thememory model. The details are unlikely to build intuition that will help one to better write ECMAScript.

More generally, even if a program is not data race free it may have predictable behaviour, so long as atomic operations are not involved in any data races and the operations that race all have the same access size. The simplest way to arrange for atomics not to be involved in races is to ensure that different memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are not used to access the same cells at the same time. Effectively, the program should treat shared memory as strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits of their values will not be mixed).

Note 2

The following are guidelines for ECMAScript implementers writing compiler transformations for programs using shared memory.

It is desirable to allow most program transformations that are valid in a single-agentsetting in a multi-agentsetting, to ensure that the performance of eachagentin a multi-agentprogram is as good as it would be in a single-agentsetting. Frequently these transformations are hard to judge. We outline some rules about program transformations that are intended to be taken as normative (in that they are implied by thememory modelor stronger than what thememory modelimplies) but which are likely not exhaustive. These rules are intended to apply to program transformations that precede the introductions of the events that make up theagent-order.

Let an agent-order slice be the subset of theagent-orderpertaining to a singleagent.

Let possible read values of a read event be the set of all values ofValueOfReadEventfor that event across all valid executions.

Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the presence of shared memory, with the following exceptions.

  • Atomics are carved in stone: Program transformations must not cause theSeqCstevents in an agent-order slice to be reordered with itsUnorderedoperations, nor itsSeqCstoperations to be reordered with each other, nor may a program transformation remove aSeqCstoperation from theagent-order.

    (In practice, the prohibition on reorderings forces a compiler to assume that everySeqCstoperation is a synchronization and included in the finalmemory-order, which it would usually have to assume anyway in the absence of inter-agentprogram analysis. It also forces the compiler to assume that every call where the callee's effects on thememory-orderare unknown may containSeqCstoperations.)

  • Reads must be stable: Any given shared memory read must only observe a single value in an execution.

    (For example, if what is semantically a single read in the program is executed multiple times then the program is subsequently allowed to observe only one of the values read. A transformation known as rematerialization can violate this rule.)

  • Writes must be stable: All observable writes to shared memory must follow from program semantics in an execution.

    (For example, a transformation may not introduce certain observable writes, such as by using read-modify-write operations on a larger location to write a smaller datum, writing a value to memory that the program could not have written, or writing a just-read value back to the location it was read from, if that location could have been overwritten by anotheragentafter the read.)

  • Possible read values must be nonempty: Program transformations cannot cause the possible read values of a shared memory read to become empty.

    (Counterintuitively, this rule in effect restricts transformations on writes, because writes have force inmemory modelinsofar as to be read by read events. For example, writes may be moved and coalesced and sometimes reordered between twoSeqCstoperations, but the transformation may not remove every write that updates a location; some write must be preserved.)

Examples of transformations that remain valid are: merging multiple non-atomic reads from the same location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that locations are different.

Note 3

The following are guidelines for ECMAScript implementers generating machine code for shared memory accesses.

For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory barriers are to be employed, such as placing barriers on both sides of a bare store or load. Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture, such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on ARM, and load-link/store-conditional instructions on Power.

Specifically, thememory modelis intended to allow code generation as follows.

  • Every atomic operation in the program is assumed to be necessary.
  • Atomic operations are never rearranged with each other or with non-atomic operations.
  • Functions are always assumed to perform atomic operations.
  • Atomic operations are never implemented as read-modify-write operations on larger data, but as non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We already assume that every platform has normal memory access operations of every interesting size.)

Naive code generation uses these patterns:

  • Regular loads and stores compile to single load and store instructions.
  • Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or store, and a full fence.
  • Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write instruction sequence, and a full fence.
  • Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and store instructions, a full fence, and a spinlock release.

That mapping is correct so long as an atomic operation on an address range does not race with a non-atomic write or with an atomic operation of different size. However, that is all we need: thememory modeleffectively demotes the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is quite strong: it allows atomic operations to be used as sequentially consistent fences, which thememory modeldoes not actually guarantee.

A number of local improvements to those basic patterns are also intended to be legal:

  • There are obvious platform-dependent improvements that remove redundant fences. For example, on x86 the fences around lock-free atomic loads and stores can always be omitted except for the fence following a store, and no fence is needed for lock-free read-modify-write instructions, as these all use LOCK-prefixed instructions. On many platforms there are fences of several strengths, and weaker fences can be used in certain contexts without destroying sequential consistency.
  • Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to have a single lock word per SharedArrayBuffer.
  • There are also more complicated platform-dependent local improvements, requiring some code analysis. For example, two back-to-back fences often have the same effect as a single fence, so if code is generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a single fence separating atomic stores can be omitted, as the fence following a store is only needed to separate the store from a subsequent load.

A Grammar Summary

A.1 Lexical Grammar

SourceCharacter::any Unicode code pointInputElementDiv::WhiteSpaceLineTerminatorCommentCommonTokenDivPunctuatorRightBracePunctuatorInputElementRegExp::WhiteSpaceLineTerminatorCommentCommonTokenRightBracePunctuatorRegularExpressionLiteralInputElementRegExpOrTemplateTail::WhiteSpaceLineTerminatorCommentCommonTokenRegularExpressionLiteralTemplateSubstitutionTailInputElementTemplateTail::WhiteSpaceLineTerminatorCommentCommonTokenDivPunctuatorTemplateSubstitutionTailWhiteSpace::<TAB><VT><FF><SP><NBSP><ZWNBSP><USP>LineTerminator::<LF><CR><LS><PS>LineTerminatorSequence::<LF><CR>[lookahead ≠<LF>]<LS><PS><CR><LF>Comment::MultiLineCommentSingleLineCommentMultiLineComment::/*MultiLineCommentCharsopt*/MultiLineCommentChars::MultiLineNotAsteriskCharMultiLineCommentCharsopt*PostAsteriskCommentCharsoptPostAsteriskCommentChars::MultiLineNotForwardSlashOrAsteriskCharMultiLineCommentCharsopt*PostAsteriskCommentCharsoptMultiLineNotAsteriskChar::SourceCharacterbut not*MultiLineNotForwardSlashOrAsteriskChar::SourceCharacterbut not one of/or*SingleLineComment:://SingleLineCommentCharsoptSingleLineCommentChars::SingleLineCommentCharSingleLineCommentCharsoptSingleLineCommentChar::SourceCharacterbut notLineTerminatorCommonToken::IdentifierNamePrivateIdentifierPunctuatorNumericLiteralStringLiteralTemplatePrivateIdentifier::#IdentifierNameIdentifierName::IdentifierStartIdentifierNameIdentifierPartIdentifierStart::UnicodeIDStart$_\UnicodeEscapeSequenceIdentifierPart::UnicodeIDContinue$\UnicodeEscapeSequence<ZWNJ><ZWJ>UnicodeIDStart::any Unicode code point with the Unicode property “ID_Start”UnicodeIDContinue::any Unicode code point with the Unicode property “ID_Continue”ReservedWord::one ofawaitbreakcasecatchclassconstcontinuedebuggerdefaultdeletedoelseenumexportextendsfalsefinallyforfunctionifimportininstanceofnewnullreturnsuperswitchthisthrowtruetrytypeofvarvoidwhilewithyieldPunctuator::OptionalChainingPunctuatorOtherPunctuatorOptionalChainingPunctuator::?.[lookahead ∉DecimalDigit]OtherPunctuator::one of{()[]....;,<><=>===!====!==+-*%**++--<<>>>>>&|^!~&&||???:=+=-=*=%=**=<<=>>=>>>=&=|=^=&&=||=??==>DivPunctuator:://=RightBracePunctuator::}NullLiteral::nullBooleanLiteral::truefalseNumericLiteralSeparator::_NumericLiteral::DecimalLiteralDecimalBigIntegerLiteralNonDecimalIntegerLiteral[+Sep]NonDecimalIntegerLiteral[+Sep]BigIntLiteralSuffixDecimalBigIntegerLiteral::0BigIntLiteralSuffixNonZeroDigitDecimalDigits[+Sep]optBigIntLiteralSuffixNonZeroDigitNumericLiteralSeparatorDecimalDigits[+Sep]BigIntLiteralSuffixNonDecimalIntegerLiteral[Sep]::BinaryIntegerLiteral[?Sep]OctalIntegerLiteral[?Sep]HexIntegerLiteral[?Sep]BigIntLiteralSuffix::nDecimalLiteral::DecimalIntegerLiteral.DecimalDigits[+Sep]optExponentPart[+Sep]opt.DecimalDigits[+Sep]ExponentPart[+Sep]optDecimalIntegerLiteralExponentPart[+Sep]optDecimalIntegerLiteral::0NonZeroDigitNonZeroDigitNumericLiteralSeparatoroptDecimalDigits[+Sep]DecimalDigits[Sep]::DecimalDigitDecimalDigits[?Sep]DecimalDigit[+Sep]DecimalDigits[+Sep]NumericLiteralSeparatorDecimalDigitDecimalDigit::one of0123456789NonZeroDigit::one of123456789ExponentPart[Sep]::ExponentIndicatorSignedInteger[?Sep]ExponentIndicator::one ofeESignedInteger[Sep]::DecimalDigits[?Sep]+DecimalDigits[?Sep]-DecimalDigits[?Sep]BinaryIntegerLiteral[Sep]::0bBinaryDigits[?Sep]0BBinaryDigits[?Sep]BinaryDigits[Sep]::BinaryDigitBinaryDigits[?Sep]BinaryDigit[+Sep]BinaryDigits[+Sep]NumericLiteralSeparatorBinaryDigitBinaryDigit::one of01OctalIntegerLiteral[Sep]::0oOctalDigits[?Sep]0OOctalDigits[?Sep]OctalDigits[Sep]::OctalDigitOctalDigits[?Sep]OctalDigit[+Sep]OctalDigits[+Sep]NumericLiteralSeparatorOctalDigitOctalDigit::one of01234567HexIntegerLiteral[Sep]::0xHexDigits[?Sep]0XHexDigits[?Sep]HexDigits[Sep]::HexDigitHexDigits[?Sep]HexDigit[+Sep]HexDigits[+Sep]NumericLiteralSeparatorHexDigitHexDigit::one of0123456789abcdefABCDEFStringLiteral::"DoubleStringCharactersopt"'SingleStringCharactersopt'DoubleStringCharacters::DoubleStringCharacterDoubleStringCharactersoptSingleStringCharacters::SingleStringCharacterSingleStringCharactersoptDoubleStringCharacter::SourceCharacterbut not one of"or\orLineTerminator<LS><PS>\EscapeSequenceLineContinuationSingleStringCharacter::SourceCharacterbut not one of'or\orLineTerminator<LS><PS>\EscapeSequenceLineContinuationLineContinuation::\LineTerminatorSequenceEscapeSequence::CharacterEscapeSequence0[lookahead ∉DecimalDigit]HexEscapeSequenceUnicodeEscapeSequenceCharacterEscapeSequence::SingleEscapeCharacterNonEscapeCharacterSingleEscapeCharacter::one of'"\bfnrtvNonEscapeCharacter::SourceCharacterbut not one ofEscapeCharacterorLineTerminatorEscapeCharacter::SingleEscapeCharacterDecimalDigitxuHexEscapeSequence::xHexDigitHexDigitUnicodeEscapeSequence::uHex4Digitsu{CodePoint}Hex4Digits::HexDigitHexDigitHexDigitHexDigitRegularExpressionLiteral::/RegularExpressionBody/RegularExpressionFlagsRegularExpressionBody::RegularExpressionFirstCharRegularExpressionCharsRegularExpressionChars::[empty]RegularExpressionCharsRegularExpressionCharRegularExpressionFirstChar::RegularExpressionNonTerminatorbut not one of*or\or/or[RegularExpressionBackslashSequenceRegularExpressionClassRegularExpressionChar::RegularExpressionNonTerminatorbut not one of\or/or[RegularExpressionBackslashSequenceRegularExpressionClassRegularExpressionBackslashSequence::\RegularExpressionNonTerminatorRegularExpressionNonTerminator::SourceCharacterbut notLineTerminatorRegularExpressionClass::[RegularExpressionClassChars]RegularExpressionClassChars::[empty]RegularExpressionClassCharsRegularExpressionClassCharRegularExpressionClassChar::RegularExpressionNonTerminatorbut not one of]or\RegularExpressionBackslashSequenceRegularExpressionFlags::[empty]RegularExpressionFlagsIdentifierPartTemplate::NoSubstitutionTemplateTemplateHeadNoSubstitutionTemplate::`TemplateCharactersopt`TemplateHead::`TemplateCharactersopt${TemplateSubstitutionTail::TemplateMiddleTemplateTailTemplateMiddle::}TemplateCharactersopt${TemplateTail::}TemplateCharactersopt`TemplateCharacters::TemplateCharacterTemplateCharactersoptTemplateCharacter::$[lookahead ≠{]\EscapeSequence\NotEscapeSequenceLineContinuationLineTerminatorSequenceSourceCharacterbut not one of`or\or$orLineTerminatorNotEscapeSequence::0DecimalDigitDecimalDigitbut not0x[lookahead ∉HexDigit]xHexDigit[lookahead ∉HexDigit]u[lookahead ∉HexDigit][lookahead ≠{]uHexDigit[lookahead ∉HexDigit]uHexDigitHexDigit[lookahead ∉HexDigit]uHexDigitHexDigitHexDigit[lookahead ∉HexDigit]u{[lookahead ∉HexDigit]u{NotCodePoint[lookahead ∉HexDigit]u{CodePoint[lookahead ∉HexDigit][lookahead ≠}]NotCodePoint::HexDigits[~Sep]but only if MV ofHexDigits> 0x10FFFFCodePoint::HexDigits[~Sep]but only if MV ofHexDigits≤ 0x10FFFF

A.2 Expressions

IdentifierReference[Yield, Await]:Identifier[~Yield]yield[~Await]awaitBindingIdentifier[Yield, Await]:IdentifieryieldawaitLabelIdentifier[Yield, Await]:Identifier[~Yield]yield[~Await]awaitIdentifier:IdentifierNamebut notReservedWordPrimaryExpression[Yield, Await]:thisIdentifierReference[?Yield, ?Await]LiteralArrayLiteral[?Yield, ?Await]ObjectLiteral[?Yield, ?Await]FunctionExpressionClassExpression[?Yield, ?Await]GeneratorExpressionAsyncFunctionExpressionAsyncGeneratorExpressionRegularExpressionLiteralTemplateLiteral[?Yield, ?Await, ~Tagged]CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]CoverParenthesizedExpressionAndArrowParameterList[Yield, Await]:(Expression[+In, ?Yield, ?Await])(Expression[+In, ?Yield, ?Await],)()(...BindingIdentifier[?Yield, ?Await])(...BindingPattern[?Yield, ?Await])(Expression[+In, ?Yield, ?Await],...BindingIdentifier[?Yield, ?Await])(Expression[+In, ?Yield, ?Await],...BindingPattern[?Yield, ?Await])

When processing an instance of the production
PrimaryExpression[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation ofCoverParenthesizedExpressionAndArrowParameterListis refined using the following grammar:

ParenthesizedExpression[Yield, Await]:(Expression[+In, ?Yield, ?Await])

 

Literal:NullLiteralBooleanLiteralNumericLiteralStringLiteralArrayLiteral[Yield, Await]:[Elisionopt][ElementList[?Yield, ?Await]][ElementList[?Yield, ?Await],Elisionopt]ElementList[Yield, Await]:ElisionoptAssignmentExpression[+In, ?Yield, ?Await]ElisionoptSpreadElement[?Yield, ?Await]ElementList[?Yield, ?Await],ElisionoptAssignmentExpression[+In, ?Yield, ?Await]ElementList[?Yield, ?Await],ElisionoptSpreadElement[?Yield, ?Await]Elision:,Elision,SpreadElement[Yield, Await]:...AssignmentExpression[+In, ?Yield, ?Await]ObjectLiteral[Yield, Await]:{}{PropertyDefinitionList[?Yield, ?Await]}{PropertyDefinitionList[?Yield, ?Await],}PropertyDefinitionList[Yield, Await]:PropertyDefinition[?Yield, ?Await]PropertyDefinitionList[?Yield, ?Await],PropertyDefinition[?Yield, ?Await]PropertyDefinition[Yield, Await]:IdentifierReference[?Yield, ?Await]CoverInitializedName[?Yield, ?Await]PropertyName[?Yield, ?Await]:AssignmentExpression[+In, ?Yield, ?Await]MethodDefinition[?Yield, ?Await]...AssignmentExpression[+In, ?Yield, ?Await]PropertyName[Yield, Await]:LiteralPropertyNameComputedPropertyName[?Yield, ?Await]LiteralPropertyName:IdentifierNameStringLiteralNumericLiteralComputedPropertyName[Yield, Await]:[AssignmentExpression[+In, ?Yield, ?Await]]CoverInitializedName[Yield, Await]:IdentifierReference[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]Initializer[In, Yield, Await]:=AssignmentExpression[?In, ?Yield, ?Await]TemplateLiteral[Yield, Await, Tagged]:NoSubstitutionTemplateSubstitutionTemplate[?Yield, ?Await, ?Tagged]SubstitutionTemplate[Yield, Await, Tagged]:TemplateHeadExpression[+In, ?Yield, ?Await]TemplateSpans[?Yield, ?Await, ?Tagged]TemplateSpans[Yield, Await, Tagged]:TemplateTailTemplateMiddleList[?Yield, ?Await, ?Tagged]TemplateTailTemplateMiddleList[Yield, Await, Tagged]:TemplateMiddleExpression[+In, ?Yield, ?Await]TemplateMiddleList[?Yield, ?Await, ?Tagged]TemplateMiddleExpression[+In, ?Yield, ?Await]MemberExpression[Yield, Await]:PrimaryExpression[?Yield, ?Await]MemberExpression[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]MemberExpression[?Yield, ?Await].IdentifierNameMemberExpression[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]SuperProperty[?Yield, ?Await]MetaPropertynewMemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]MemberExpression[?Yield, ?Await].PrivateIdentifierSuperProperty[Yield, Await]:super[Expression[+In, ?Yield, ?Await]]super.IdentifierNameMetaProperty:NewTargetImportMetaNewTarget:new.targetImportMeta:import.metaNewExpression[Yield, Await]:MemberExpression[?Yield, ?Await]newNewExpression[?Yield, ?Await]CallExpression[Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await]SuperCall[?Yield, ?Await]ImportCall[?Yield, ?Await]CallExpression[?Yield, ?Await]Arguments[?Yield, ?Await]CallExpression[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]CallExpression[?Yield, ?Await].IdentifierNameCallExpression[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]CallExpression[?Yield, ?Await].PrivateIdentifier

When processing an instance of the production
CallExpression[Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await]
the interpretation ofCoverCallExpressionAndAsyncArrowHeadis refined using the following grammar:

CallMemberExpression[Yield, Await]:MemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]

 

SuperCall[Yield, Await]:superArguments[?Yield, ?Await]ImportCall[Yield, Await]:import(AssignmentExpression[+In, ?Yield, ?Await])Arguments[Yield, Await]:()(ArgumentList[?Yield, ?Await])(ArgumentList[?Yield, ?Await],)ArgumentList[Yield, Await]:AssignmentExpression[+In, ?Yield, ?Await]...AssignmentExpression[+In, ?Yield, ?Await]ArgumentList[?Yield, ?Await],AssignmentExpression[+In, ?Yield, ?Await]ArgumentList[?Yield, ?Await],...AssignmentExpression[+In, ?Yield, ?Await]OptionalExpression[Yield, Await]:MemberExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]CallExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]OptionalExpression[?Yield, ?Await]OptionalChain[?Yield, ?Await]OptionalChain[Yield, Await]:?.Arguments[?Yield, ?Await]?.[Expression[+In, ?Yield, ?Await]]?.IdentifierName?.TemplateLiteral[?Yield, ?Await, +Tagged]?.PrivateIdentifierOptionalChain[?Yield, ?Await]Arguments[?Yield, ?Await]OptionalChain[?Yield, ?Await][Expression[+In, ?Yield, ?Await]]OptionalChain[?Yield, ?Await].IdentifierNameOptionalChain[?Yield, ?Await]TemplateLiteral[?Yield, ?Await, +Tagged]OptionalChain[?Yield, ?Await].PrivateIdentifierLeftHandSideExpression[Yield, Await]:NewExpression[?Yield, ?Await]CallExpression[?Yield, ?Await]OptionalExpression[?Yield, ?Await]UpdateExpression[Yield, Await]:LeftHandSideExpression[?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]++LeftHandSideExpression[?Yield, ?Await][noLineTerminatorhere]--++UnaryExpression[?Yield, ?Await]--UnaryExpression[?Yield, ?Await]UnaryExpression[Yield, Await]:UpdateExpression[?Yield, ?Await]deleteUnaryExpression[?Yield, ?Await]voidUnaryExpression[?Yield, ?Await]typeofUnaryExpression[?Yield, ?Await]+UnaryExpression[?Yield, ?Await]-UnaryExpression[?Yield, ?Await]~UnaryExpression[?Yield, ?Await]!UnaryExpression[?Yield, ?Await][+Await]AwaitExpression[?Yield]ExponentiationExpression[Yield, Await]:UnaryExpression[?Yield, ?Await]UpdateExpression[?Yield, ?Await]**ExponentiationExpression[?Yield, ?Await]MultiplicativeExpression[Yield, Await]:ExponentiationExpression[?Yield, ?Await]MultiplicativeExpression[?Yield, ?Await]MultiplicativeOperatorExponentiationExpression[?Yield, ?Await]MultiplicativeOperator:one of*/%AdditiveExpression[Yield, Await]:MultiplicativeExpression[?Yield, ?Await]AdditiveExpression[?Yield, ?Await]+MultiplicativeExpression[?Yield, ?Await]AdditiveExpression[?Yield, ?Await]-MultiplicativeExpression[?Yield, ?Await]ShiftExpression[Yield, Await]:AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]<<AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]>>AdditiveExpression[?Yield, ?Await]ShiftExpression[?Yield, ?Await]>>>AdditiveExpression[?Yield, ?Await]RelationalExpression[In, Yield, Await]:ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]<ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]>ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]<=ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]>=ShiftExpression[?Yield, ?Await]RelationalExpression[?In, ?Yield, ?Await]instanceofShiftExpression[?Yield, ?Await][+In]RelationalExpression[+In, ?Yield, ?Await]inShiftExpression[?Yield, ?Await][+In]PrivateIdentifierinShiftExpression[?Yield, ?Await]EqualityExpression[In, Yield, Await]:RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]==RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]!=RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]===RelationalExpression[?In, ?Yield, ?Await]EqualityExpression[?In, ?Yield, ?Await]!==RelationalExpression[?In, ?Yield, ?Await]BitwiseANDExpression[In, Yield, Await]:EqualityExpression[?In, ?Yield, ?Await]BitwiseANDExpression[?In, ?Yield, ?Await]&EqualityExpression[?In, ?Yield, ?Await]BitwiseXORExpression[In, Yield, Await]:BitwiseANDExpression[?In, ?Yield, ?Await]BitwiseXORExpression[?In, ?Yield, ?Await]^BitwiseANDExpression[?In, ?Yield, ?Await]BitwiseORExpression[In, Yield, Await]:BitwiseXORExpression[?In, ?Yield, ?Await]BitwiseORExpression[?In, ?Yield, ?Await]|BitwiseXORExpression[?In, ?Yield, ?Await]LogicalANDExpression[In, Yield, Await]:BitwiseORExpression[?In, ?Yield, ?Await]LogicalANDExpression[?In, ?Yield, ?Await]&&BitwiseORExpression[?In, ?Yield, ?Await]LogicalORExpression[In, Yield, Await]:LogicalANDExpression[?In, ?Yield, ?Await]LogicalORExpression[?In, ?Yield, ?Await]||LogicalANDExpression[?In, ?Yield, ?Await]CoalesceExpression[In, Yield, Await]:CoalesceExpressionHead[?In, ?Yield, ?Await]??BitwiseORExpression[?In, ?Yield, ?Await]CoalesceExpressionHead[In, Yield, Await]:CoalesceExpression[?In, ?Yield, ?Await]BitwiseORExpression[?In, ?Yield, ?Await]ShortCircuitExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await]CoalesceExpression[?In, ?Yield, ?Await]ConditionalExpression[In, Yield, Await]:ShortCircuitExpression[?In, ?Yield, ?Await]ShortCircuitExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[?In, ?Yield, ?Await]AssignmentExpression[In, Yield, Await]:ConditionalExpression[?In, ?Yield, ?Await][+Yield]YieldExpression[?In, ?Await]ArrowFunction[?In, ?Yield, ?Await]AsyncArrowFunction[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]AssignmentOperatorAssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]&&=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]||=AssignmentExpression[?In, ?Yield, ?Await]LeftHandSideExpression[?Yield, ?Await]??=AssignmentExpression[?In, ?Yield, ?Await]AssignmentOperator:one of*=/=%=+=-=<<=>>=>>>=&=^=|=**=

In certain circumstances when processing an instance of the production
AssignmentExpression[In, Yield, Await]:LeftHandSideExpression[?Yield, ?Await]=AssignmentExpression[?In, ?Yield, ?Await]
the interpretation ofLeftHandSideExpressionis refined using the following grammar:

AssignmentPattern[Yield, Await]:ObjectAssignmentPattern[?Yield, ?Await]ArrayAssignmentPattern[?Yield, ?Await]ObjectAssignmentPattern[Yield, Await]:{}{AssignmentRestProperty[?Yield, ?Await]}{AssignmentPropertyList[?Yield, ?Await]}{AssignmentPropertyList[?Yield, ?Await],AssignmentRestProperty[?Yield, ?Await]opt}ArrayAssignmentPattern[Yield, Await]:[ElisionoptAssignmentRestElement[?Yield, ?Await]opt][AssignmentElementList[?Yield, ?Await]][AssignmentElementList[?Yield, ?Await],ElisionoptAssignmentRestElement[?Yield, ?Await]opt]AssignmentRestProperty[Yield, Await]:...DestructuringAssignmentTarget[?Yield, ?Await]AssignmentPropertyList[Yield, Await]:AssignmentProperty[?Yield, ?Await]AssignmentPropertyList[?Yield, ?Await],AssignmentProperty[?Yield, ?Await]AssignmentElementList[Yield, Await]:AssignmentElisionElement[?Yield, ?Await]AssignmentElementList[?Yield, ?Await],AssignmentElisionElement[?Yield, ?Await]AssignmentElisionElement[Yield, Await]:ElisionoptAssignmentElement[?Yield, ?Await]AssignmentProperty[Yield, Await]:IdentifierReference[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optPropertyName[?Yield, ?Await]:AssignmentElement[?Yield, ?Await]AssignmentElement[Yield, Await]:DestructuringAssignmentTarget[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optAssignmentRestElement[Yield, Await]:...DestructuringAssignmentTarget[?Yield, ?Await]DestructuringAssignmentTarget[Yield, Await]:LeftHandSideExpression[?Yield, ?Await]

 

Expression[In, Yield, Await]:AssignmentExpression[?In, ?Yield, ?Await]Expression[?In, ?Yield, ?Await],AssignmentExpression[?In, ?Yield, ?Await]

A.3 Statements

Statement[Yield, Await, Return]:BlockStatement[?Yield, ?Await, ?Return]VariableStatement[?Yield, ?Await]EmptyStatementExpressionStatement[?Yield, ?Await]IfStatement[?Yield, ?Await, ?Return]BreakableStatement[?Yield, ?Await, ?Return]ContinueStatement[?Yield, ?Await]BreakStatement[?Yield, ?Await][+Return]ReturnStatement[?Yield, ?Await]WithStatement[?Yield, ?Await, ?Return]LabelledStatement[?Yield, ?Await, ?Return]ThrowStatement[?Yield, ?Await]TryStatement[?Yield, ?Await, ?Return]DebuggerStatementDeclaration[Yield, Await]:HoistableDeclaration[?Yield, ?Await, ~Default]ClassDeclaration[?Yield, ?Await, ~Default]LexicalDeclaration[+In, ?Yield, ?Await]HoistableDeclaration[Yield, Await, Default]:FunctionDeclaration[?Yield, ?Await, ?Default]GeneratorDeclaration[?Yield, ?Await, ?Default]AsyncFunctionDeclaration[?Yield, ?Await, ?Default]AsyncGeneratorDeclaration[?Yield, ?Await, ?Default]BreakableStatement[Yield, Await, Return]:IterationStatement[?Yield, ?Await, ?Return]SwitchStatement[?Yield, ?Await, ?Return]BlockStatement[Yield, Await, Return]:Block[?Yield, ?Await, ?Return]Block[Yield, Await, Return]:{StatementList[?Yield, ?Await, ?Return]opt}StatementList[Yield, Await, Return]:StatementListItem[?Yield, ?Await, ?Return]StatementList[?Yield, ?Await, ?Return]StatementListItem[?Yield, ?Await, ?Return]StatementListItem[Yield, Await, Return]:Statement[?Yield, ?Await, ?Return]Declaration[?Yield, ?Await]LexicalDeclaration[In, Yield, Await]:LetOrConstBindingList[?In, ?Yield, ?Await];LetOrConst:letconstBindingList[In, Yield, Await]:LexicalBinding[?In, ?Yield, ?Await]BindingList[?In, ?Yield, ?Await],LexicalBinding[?In, ?Yield, ?Await]LexicalBinding[In, Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]optBindingPattern[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]VariableStatement[Yield, Await]:varVariableDeclarationList[+In, ?Yield, ?Await];VariableDeclarationList[In, Yield, Await]:VariableDeclaration[?In, ?Yield, ?Await]VariableDeclarationList[?In, ?Yield, ?Await],VariableDeclaration[?In, ?Yield, ?Await]VariableDeclaration[In, Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]optBindingPattern[?Yield, ?Await]Initializer[?In, ?Yield, ?Await]BindingPattern[Yield, Await]:ObjectBindingPattern[?Yield, ?Await]ArrayBindingPattern[?Yield, ?Await]ObjectBindingPattern[Yield, Await]:{}{BindingRestProperty[?Yield, ?Await]}{BindingPropertyList[?Yield, ?Await]}{BindingPropertyList[?Yield, ?Await],BindingRestProperty[?Yield, ?Await]opt}ArrayBindingPattern[Yield, Await]:[ElisionoptBindingRestElement[?Yield, ?Await]opt][BindingElementList[?Yield, ?Await]][BindingElementList[?Yield, ?Await],ElisionoptBindingRestElement[?Yield, ?Await]opt]BindingRestProperty[Yield, Await]:...BindingIdentifier[?Yield, ?Await]BindingPropertyList[Yield, Await]:BindingProperty[?Yield, ?Await]BindingPropertyList[?Yield, ?Await],BindingProperty[?Yield, ?Await]BindingElementList[Yield, Await]:BindingElisionElement[?Yield, ?Await]BindingElementList[?Yield, ?Await],BindingElisionElement[?Yield, ?Await]BindingElisionElement[Yield, Await]:ElisionoptBindingElement[?Yield, ?Await]BindingProperty[Yield, Await]:SingleNameBinding[?Yield, ?Await]PropertyName[?Yield, ?Await]:BindingElement[?Yield, ?Await]BindingElement[Yield, Await]:SingleNameBinding[?Yield, ?Await]BindingPattern[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optSingleNameBinding[Yield, Await]:BindingIdentifier[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optBindingRestElement[Yield, Await]:...BindingIdentifier[?Yield, ?Await]...BindingPattern[?Yield, ?Await]EmptyStatement:;ExpressionStatement[Yield, Await]:[lookahead ∉ {{,function,async[noLineTerminatorhere]function,class,let[}]Expression[+In, ?Yield, ?Await];IfStatement[Yield, Await, Return]:if(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]elseStatement[?Yield, ?Await, ?Return]if(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][lookahead ≠else]IterationStatement[Yield, Await, Return]:DoWhileStatement[?Yield, ?Await, ?Return]WhileStatement[?Yield, ?Await, ?Return]ForStatement[?Yield, ?Await, ?Return]ForInOfStatement[?Yield, ?Await, ?Return]DoWhileStatement[Yield, Await, Return]:doStatement[?Yield, ?Await, ?Return]while(Expression[+In, ?Yield, ?Await]);WhileStatement[Yield, Await, Return]:while(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]ForStatement[Yield, Await, Return]:for([lookahead ≠let[]Expression[~In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]for(varVariableDeclarationList[~In, ?Yield, ?Await];Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]for(LexicalDeclaration[~In, ?Yield, ?Await]Expression[+In, ?Yield, ?Await]opt;Expression[+In, ?Yield, ?Await]opt)Statement[?Yield, ?Await, ?Return]ForInOfStatement[Yield, Await, Return]:for([lookahead ≠let[]LeftHandSideExpression[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(varForBinding[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(ForDeclaration[?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for([lookahead ∉ {let,asyncof}]LeftHandSideExpression[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(varForBinding[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]for(ForDeclaration[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait([lookahead ≠let]LeftHandSideExpression[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait(varForBinding[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return][+Await]forawait(ForDeclaration[?Yield, ?Await]ofAssignmentExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]ForDeclaration[Yield, Await]:LetOrConstForBinding[?Yield, ?Await]ForBinding[Yield, Await]:BindingIdentifier[?Yield, ?Await]BindingPattern[?Yield, ?Await]ContinueStatement[Yield, Await]:continue;continue[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];BreakStatement[Yield, Await]:break;break[noLineTerminatorhere]LabelIdentifier[?Yield, ?Await];ReturnStatement[Yield, Await]:return;return[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];WithStatement[Yield, Await, Return]:with(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]SwitchStatement[Yield, Await, Return]:switch(Expression[+In, ?Yield, ?Await])CaseBlock[?Yield, ?Await, ?Return]CaseBlock[Yield, Await, Return]:{CaseClauses[?Yield, ?Await, ?Return]opt}{CaseClauses[?Yield, ?Await, ?Return]optDefaultClause[?Yield, ?Await, ?Return]CaseClauses[?Yield, ?Await, ?Return]opt}CaseClauses[Yield, Await, Return]:CaseClause[?Yield, ?Await, ?Return]CaseClauses[?Yield, ?Await, ?Return]CaseClause[?Yield, ?Await, ?Return]CaseClause[Yield, Await, Return]:caseExpression[+In, ?Yield, ?Await]:StatementList[?Yield, ?Await, ?Return]optDefaultClause[Yield, Await, Return]:default:StatementList[?Yield, ?Await, ?Return]optLabelledStatement[Yield, Await, Return]:LabelIdentifier[?Yield, ?Await]:LabelledItem[?Yield, ?Await, ?Return]LabelledItem[Yield, Await, Return]:Statement[?Yield, ?Await, ?Return]FunctionDeclaration[?Yield, ?Await, ~Default]ThrowStatement[Yield, Await]:throw[noLineTerminatorhere]Expression[+In, ?Yield, ?Await];TryStatement[Yield, Await, Return]:tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]tryBlock[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return]tryBlock[?Yield, ?Await, ?Return]Catch[?Yield, ?Await, ?Return]Finally[?Yield, ?Await, ?Return]Catch[Yield, Await, Return]:catch(CatchParameter[?Yield, ?Await])Block[?Yield, ?Await, ?Return]catchBlock[?Yield, ?Await, ?Return]Finally[Yield, Await, Return]:finallyBlock[?Yield, ?Await, ?Return]CatchParameter[Yield, Await]:BindingIdentifier[?Yield, ?Await]BindingPattern[?Yield, ?Await]DebuggerStatement:debugger;

A.4 Functions and Classes

UniqueFormalParameters[Yield, Await]:FormalParameters[?Yield, ?Await]FormalParameters[Yield, Await]:[empty]FunctionRestParameter[?Yield, ?Await]FormalParameterList[?Yield, ?Await]FormalParameterList[?Yield, ?Await],FormalParameterList[?Yield, ?Await],FunctionRestParameter[?Yield, ?Await]FormalParameterList[Yield, Await]:FormalParameter[?Yield, ?Await]FormalParameterList[?Yield, ?Await],FormalParameter[?Yield, ?Await]FunctionRestParameter[Yield, Await]:BindingRestElement[?Yield, ?Await]FormalParameter[Yield, Await]:BindingElement[?Yield, ?Await]FunctionDeclaration[Yield, Await, Default]:functionBindingIdentifier[?Yield, ?Await](FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}[+Default]function(FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}FunctionExpression:functionBindingIdentifier[~Yield, ~Await]opt(FormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}FunctionBody[Yield, Await]:FunctionStatementList[?Yield, ?Await]FunctionStatementList[Yield, Await]:StatementList[?Yield, ?Await, +Return]optArrowFunction[In, Yield, Await]:ArrowParameters[?Yield, ?Await][noLineTerminatorhere]=>ConciseBody[?In]ArrowParameters[Yield, Await]:BindingIdentifier[?Yield, ?Await]CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]ConciseBody[In]:[lookahead ≠{]ExpressionBody[?In, ~Await]{FunctionBody[~Yield, ~Await]}ExpressionBody[In, Await]:AssignmentExpression[?In, ~Yield, ?Await]

When processing an instance of the production
ArrowParameters[Yield, Await]:CoverParenthesizedExpressionAndArrowParameterList[?Yield, ?Await]
the interpretation ofCoverParenthesizedExpressionAndArrowParameterListis refined using the following grammar:

ArrowFormalParameters[Yield, Await]:(UniqueFormalParameters[?Yield, ?Await])

 

AsyncArrowFunction[In, Yield, Await]:async[noLineTerminatorhere]AsyncArrowBindingIdentifier[?Yield][noLineTerminatorhere]=>AsyncConciseBody[?In]CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await][noLineTerminatorhere]=>AsyncConciseBody[?In]AsyncConciseBody[In]:[lookahead ≠{]ExpressionBody[?In, +Await]{AsyncFunctionBody}AsyncArrowBindingIdentifier[Yield]:BindingIdentifier[?Yield, +Await]CoverCallExpressionAndAsyncArrowHead[Yield, Await]:MemberExpression[?Yield, ?Await]Arguments[?Yield, ?Await]

When processing an instance of the production
AsyncArrowFunction[In, Yield, Await]:CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await][noLineTerminatorhere]=>AsyncConciseBody[?In]
the interpretation ofCoverCallExpressionAndAsyncArrowHeadis refined using the following grammar:

AsyncArrowHead:async[noLineTerminatorhere]ArrowFormalParameters[~Yield, +Await]

 

MethodDefinition[Yield, Await]:ClassElementName[?Yield, ?Await](UniqueFormalParameters[~Yield, ~Await]){FunctionBody[~Yield, ~Await]}GeneratorMethod[?Yield, ?Await]AsyncMethod[?Yield, ?Await]AsyncGeneratorMethod[?Yield, ?Await]getClassElementName[?Yield, ?Await](){FunctionBody[~Yield, ~Await]}setClassElementName[?Yield, ?Await](PropertySetParameterList){FunctionBody[~Yield, ~Await]}PropertySetParameterList:FormalParameter[~Yield, ~Await]GeneratorMethod[Yield, Await]:*ClassElementName[?Yield, ?Await](UniqueFormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorDeclaration[Yield, Await, Default]:function*BindingIdentifier[?Yield, ?Await](FormalParameters[+Yield, ~Await]){GeneratorBody}[+Default]function*(FormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorExpression:function*BindingIdentifier[+Yield, ~Await]opt(FormalParameters[+Yield, ~Await]){GeneratorBody}GeneratorBody:FunctionBody[+Yield, ~Await]YieldExpression[In, Await]:yieldyield[noLineTerminatorhere]AssignmentExpression[?In, +Yield, ?Await]yield[noLineTerminatorhere]*AssignmentExpression[?In, +Yield, ?Await]AsyncGeneratorMethod[Yield, Await]:async[noLineTerminatorhere]*ClassElementName[?Yield, ?Await](UniqueFormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorDeclaration[Yield, Await, Default]:async[noLineTerminatorhere]function*BindingIdentifier[?Yield, ?Await](FormalParameters[+Yield, +Await]){AsyncGeneratorBody}[+Default]async[noLineTerminatorhere]function*(FormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorExpression:async[noLineTerminatorhere]function*BindingIdentifier[+Yield, +Await]opt(FormalParameters[+Yield, +Await]){AsyncGeneratorBody}AsyncGeneratorBody:FunctionBody[+Yield, +Await]AsyncFunctionDeclaration[Yield, Await, Default]:async[noLineTerminatorhere]functionBindingIdentifier[?Yield, ?Await](FormalParameters[~Yield, +Await]){AsyncFunctionBody}[+Default]async[noLineTerminatorhere]function(FormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncFunctionExpression:async[noLineTerminatorhere]functionBindingIdentifier[~Yield, +Await]opt(FormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncMethod[Yield, Await]:async[noLineTerminatorhere]ClassElementName[?Yield, ?Await](UniqueFormalParameters[~Yield, +Await]){AsyncFunctionBody}AsyncFunctionBody:FunctionBody[~Yield, +Await]AwaitExpression[Yield]:awaitUnaryExpression[?Yield, +Await]ClassDeclaration[Yield, Await, Default]:classBindingIdentifier[?Yield, ?Await]ClassTail[?Yield, ?Await][+Default]classClassTail[?Yield, ?Await]ClassExpression[Yield, Await]:classBindingIdentifier[?Yield, ?Await]optClassTail[?Yield, ?Await]ClassTail[Yield, Await]:ClassHeritage[?Yield, ?Await]opt{ClassBody[?Yield, ?Await]opt}ClassHeritage[Yield, Await]:extendsLeftHandSideExpression[?Yield, ?Await]ClassBody[Yield, Await]:ClassElementList[?Yield, ?Await]ClassElementList[Yield, Await]:ClassElement[?Yield, ?Await]ClassElementList[?Yield, ?Await]ClassElement[?Yield, ?Await]ClassElement[Yield, Await]:MethodDefinition[?Yield, ?Await]staticMethodDefinition[?Yield, ?Await]FieldDefinition[?Yield, ?Await];staticFieldDefinition[?Yield, ?Await];;FieldDefinition[Yield, Await]:ClassElementName[?Yield, ?Await]Initializer[+In, ?Yield, ?Await]optClassElementName[Yield, Await]:PropertyName[?Yield, ?Await]PrivateIdentifier

A.5 Scripts and Modules

Script:ScriptBodyoptScriptBody:StatementList[~Yield, ~Await, ~Return]Module:ModuleBodyoptModuleBody:ModuleItemListModuleItemList:ModuleItemModuleItemListModuleItemModuleItem:ImportDeclarationExportDeclarationStatementListItem[~Yield, ~Await, ~Return]ImportDeclaration:importImportClauseFromClause;importModuleSpecifier;ImportClause:ImportedDefaultBindingNameSpaceImportNamedImportsImportedDefaultBinding,NameSpaceImportImportedDefaultBinding,NamedImportsImportedDefaultBinding:ImportedBindingNameSpaceImport:*asImportedBindingNamedImports:{}{ImportsList}{ImportsList,}FromClause:fromModuleSpecifierImportsList:ImportSpecifierImportsList,ImportSpecifierImportSpecifier:ImportedBindingIdentifierNameasImportedBindingModuleSpecifier:StringLiteralImportedBinding:BindingIdentifier[~Yield, ~Await]ExportDeclaration:exportExportFromClauseFromClause;exportNamedExports;exportVariableStatement[~Yield, ~Await]exportDeclaration[~Yield, ~Await]exportdefaultHoistableDeclaration[~Yield, ~Await, +Default]exportdefaultClassDeclaration[~Yield, ~Await, +Default]exportdefault[lookahead ∉ {function,async[noLineTerminatorhere]function,class}]AssignmentExpression[+In, ~Yield, ~Await];ExportFromClause:**asIdentifierNameNamedExportsNamedExports:{}{ExportsList}{ExportsList,}ExportsList:ExportSpecifierExportsList,ExportSpecifierExportSpecifier:IdentifierNameIdentifierNameasIdentifierName

A.6 Number Conversions

StringNumericLiteral:::StrWhiteSpaceoptStrWhiteSpaceoptStrNumericLiteralStrWhiteSpaceoptStrWhiteSpace:::StrWhiteSpaceCharStrWhiteSpaceoptStrWhiteSpaceChar:::WhiteSpaceLineTerminatorStrNumericLiteral:::StrDecimalLiteralNonDecimalIntegerLiteral[~Sep]StrDecimalLiteral:::StrUnsignedDecimalLiteral+StrUnsignedDecimalLiteral-StrUnsignedDecimalLiteralStrUnsignedDecimalLiteral:::InfinityDecimalDigits[~Sep].DecimalDigits[~Sep]optExponentPart[~Sep]opt.DecimalDigits[~Sep]ExponentPart[~Sep]optDecimalDigits[~Sep]ExponentPart[~Sep]opt

All grammar symbols not explicitly defined by theStringNumericLiteralgrammar have the definitions used in theLexical Grammar for numeric literals.

A.7 Universal Resource Identifier Character Classes

uri:::uriCharactersopturiCharacters:::uriCharacteruriCharactersopturiCharacter:::uriReserveduriUnescapeduriEscapeduriReserved:::one of;/?:@&=+$,uriUnescaped:::uriAlphaDecimalDigituriMarkuriEscaped:::%HexDigitHexDigituriAlpha:::one ofabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZuriMark:::one of-_.!~*'()

A.8 Regular Expressions

Pattern[U, N]::Disjunction[?U, ?N]Disjunction[U, N]::Alternative[?U, ?N]Alternative[?U, ?N]|Disjunction[?U, ?N]Alternative[U, N]::[empty]Alternative[?U, ?N]Term[?U, ?N]Term[U, N]::Assertion[?U, ?N]Atom[?U, ?N]Atom[?U, ?N]QuantifierAssertion[U, N]::^$\b\B(?=Disjunction[?U, ?N])(?!Disjunction[?U, ?N])(?<=Disjunction[?U, ?N])(?<!Disjunction[?U, ?N])Quantifier::QuantifierPrefixQuantifierPrefix?QuantifierPrefix::*+?{DecimalDigits[~Sep]}{DecimalDigits[~Sep],}{DecimalDigits[~Sep],DecimalDigits[~Sep]}Atom[U, N]::PatternCharacter.\AtomEscape[?U, ?N]CharacterClass[?U](GroupSpecifier[?U]Disjunction[?U, ?N])(?:Disjunction[?U, ?N])SyntaxCharacter::one of^$\.*+?()[]{}|PatternCharacter::SourceCharacterbut notSyntaxCharacterAtomEscape[U, N]::DecimalEscapeCharacterClassEscape[?U]CharacterEscape[?U][+N]kGroupName[?U]CharacterEscape[U]::ControlEscapecControlLetter0[lookahead ∉DecimalDigit]HexEscapeSequenceRegExpUnicodeEscapeSequence[?U]IdentityEscape[?U]ControlEscape::one offnrtvControlLetter::one ofabcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZGroupSpecifier[U]::[empty]?GroupName[?U]GroupName[U]::<RegExpIdentifierName[?U]>RegExpIdentifierName[U]::RegExpIdentifierStart[?U]RegExpIdentifierName[?U]RegExpIdentifierPart[?U]RegExpIdentifierStart[U]::UnicodeIDStart$_\RegExpUnicodeEscapeSequence[+U][~U]UnicodeLeadSurrogateUnicodeTrailSurrogateRegExpIdentifierPart[U]::UnicodeIDContinue$\RegExpUnicodeEscapeSequence[+U][~U]UnicodeLeadSurrogateUnicodeTrailSurrogate<ZWNJ><ZWJ>RegExpUnicodeEscapeSequence[U]::[+U]uHexLeadSurrogate\uHexTrailSurrogate[+U]uHexLeadSurrogate[+U]uHexTrailSurrogate[+U]uHexNonSurrogate[~U]uHex4Digits[+U]u{CodePoint}UnicodeLeadSurrogate::any Unicode code point in the inclusive range 0xD800 to 0xDBFFUnicodeTrailSurrogate::any Unicode code point in the inclusive range 0xDC00 to 0xDFFF

Each \uHexTrailSurrogatefor which the choice of associated uHexLeadSurrogateis ambiguous shall be associated with the nearest possible uHexLeadSurrogatethat would otherwise have no corresponding \uHexTrailSurrogate.

 

HexLeadSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis in the inclusive range 0xD800 to 0xDBFFHexTrailSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis in the inclusive range 0xDC00 to 0xDFFFHexNonSurrogate::Hex4Digitsbut only if the MV ofHex4Digitsis not in the inclusive range 0xD800 to 0xDFFFIdentityEscape[U]::[+U]SyntaxCharacter[+U]/[~U]SourceCharacterbut notUnicodeIDContinueDecimalEscape::NonZeroDigitDecimalDigits[~Sep]opt[lookahead ∉DecimalDigit]CharacterClassEscape[U]::dDsSwW[+U]p{UnicodePropertyValueExpression}[+U]P{UnicodePropertyValueExpression}UnicodePropertyValueExpression::UnicodePropertyName=UnicodePropertyValueLoneUnicodePropertyNameOrValueUnicodePropertyName::UnicodePropertyNameCharactersUnicodePropertyNameCharacters::UnicodePropertyNameCharacterUnicodePropertyNameCharactersoptUnicodePropertyValue::UnicodePropertyValueCharactersLoneUnicodePropertyNameOrValue::UnicodePropertyValueCharactersUnicodePropertyValueCharacters::UnicodePropertyValueCharacterUnicodePropertyValueCharactersoptUnicodePropertyValueCharacter::UnicodePropertyNameCharacterDecimalDigitUnicodePropertyNameCharacter::ControlLetter_CharacterClass[U]::[[lookahead ≠^]ClassRanges[?U]][^ClassRanges[?U]]ClassRanges[U]::[empty]NonemptyClassRanges[?U]NonemptyClassRanges[U]::ClassAtom[?U]ClassAtom[?U]NonemptyClassRangesNoDash[?U]ClassAtom[?U]-ClassAtom[?U]ClassRanges[?U]NonemptyClassRangesNoDash[U]::ClassAtom[?U]ClassAtomNoDash[?U]NonemptyClassRangesNoDash[?U]ClassAtomNoDash[?U]-ClassAtom[?U]ClassRanges[?U]ClassAtom[U]::-ClassAtomNoDash[?U]ClassAtomNoDash[U]::SourceCharacterbut not one of\or]or-\ClassEscape[?U]ClassEscape[U]::b[+U]-CharacterClassEscape[?U]CharacterEscape[?U]

B Additional ECMAScript Features for Web Browsers

The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScripthostis a web browser. The content of this annex is normative but optional if the ECMAScripthostis not a web browser.

Note

This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the language features and behaviours specified in this annex have one or more undesirable characteristics and in the absence of legacy usage would be removed from this specification. However, the usage of these features by large numbers of existing web pages means that web browsers must continue to support them. The specifications in this annex define the requirements for interoperable implementations of these legacy features.

These features are not considered part of the core ECMAScript language. Programmers should not use or assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript implementations are discouraged from implementing these features unless the implementation is part of a web browser or is required to run the same legacy ECMAScript code that web browsers encounter.

B.1 Additional Syntax

B.1.1 Numeric Literals

The syntax and semantics of12.8.3is extended as follows except that this extension is not allowed forstrict mode code:

Syntax

NumericLiteral::DecimalLiteralDecimalBigIntegerLiteralNonDecimalIntegerLiteral[+Sep]NonDecimalIntegerLiteral[+Sep]BigIntLiteralSuffixLegacyOctalIntegerLiteralLegacyOctalIntegerLiteral::0OctalDigitLegacyOctalIntegerLiteralOctalDigitDecimalIntegerLiteral::0NonZeroDigitNonZeroDigitNumericLiteralSeparatoroptDecimalDigits[+Sep]NonOctalDecimalIntegerLiteralNonOctalDecimalIntegerLiteral::0NonOctalDigitLegacyOctalLikeDecimalIntegerLiteralNonOctalDigitNonOctalDecimalIntegerLiteralDecimalDigitLegacyOctalLikeDecimalIntegerLiteral::0OctalDigitLegacyOctalLikeDecimalIntegerLiteralOctalDigitNonOctalDigit::one of89

B.1.1.1 Static Semantics

B.1.2 String Literals

The syntax and semantics of12.8.4is extended as follows except that this extension is not allowed forstrict mode code:

Syntax

EscapeSequence::CharacterEscapeSequenceLegacyOctalEscapeSequenceNonOctalDecimalEscapeSequenceHexEscapeSequenceUnicodeEscapeSequenceLegacyOctalEscapeSequence::OctalDigit[lookahead ∉OctalDigit]ZeroToThreeOctalDigit[lookahead ∉OctalDigit]FourToSevenOctalDigitZeroToThreeOctalDigitOctalDigitZeroToThree::one of0123FourToSeven::one of4567NonOctalDecimalEscapeSequence::one of89

This definition ofEscapeSequenceis not used in strict mode or when parsingTemplateCharacter.

Note

It is possible for string literals to precede aUse Strict Directivethat places the enclosing code instrict mode, and implementations must take care to not use this extended definition ofEscapeSequencewith such literals. For example, attempting to parse the following source text must fail:

function invalid() { "\7"; "use strict"; }

B.1.2.1 Static Semantics

B.1.3 HTML-like Comments

The syntax and semantics of12.4is extended as follows except that this extension is not allowed when parsing source code using thegoal symbolModule:

Syntax

Comment::MultiLineCommentSingleLineCommentSingleLineHTMLOpenCommentSingleLineHTMLCloseCommentSingleLineDelimitedCommentMultiLineComment::/*FirstCommentLineoptLineTerminatorMultiLineCommentCharsopt*/HTMLCloseCommentoptFirstCommentLine::SingleLineDelimitedCommentCharsSingleLineHTMLOpenComment::<!--SingleLineCommentCharsoptSingleLineHTMLCloseComment::LineTerminatorSequenceHTMLCloseCommentSingleLineDelimitedComment::/*SingleLineDelimitedCommentCharsopt*/HTMLCloseComment::WhiteSpaceSequenceoptSingleLineDelimitedCommentSequenceopt-->SingleLineCommentCharsoptSingleLineDelimitedCommentChars::SingleLineNotAsteriskCharSingleLineDelimitedCommentCharsopt*SingleLinePostAsteriskCommentCharsoptSingleLineNotAsteriskChar::SourceCharacterbut not one of*orLineTerminatorSingleLinePostAsteriskCommentChars::SingleLineNotForwardSlashOrAsteriskCharSingleLineDelimitedCommentCharsopt*SingleLinePostAsteriskCommentCharsoptSingleLineNotForwardSlashOrAsteriskChar::SourceCharacterbut not one of/or*orLineTerminatorWhiteSpaceSequence::WhiteSpaceWhiteSpaceSequenceoptSingleLineDelimitedCommentSequence::SingleLineDelimitedCommentWhiteSpaceSequenceoptSingleLineDelimitedCommentSequenceopt

Similar to aMultiLineCommentthat contains a line terminator code point, aSingleLineHTMLCloseCommentis considered to be aLineTerminatorfor purposes of parsing by the syntactic grammar.

B.1.4 Regular Expressions Patterns

The syntax of22.2.1is modified and extended as follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by contextual information. When parsing using the following grammar, each alternative is considered only if previous production alternatives do not match.

This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The following grammar extensions include productions parameterized with the [U] parameter. However, none of these extensions change the syntax of Unicode patterns recognized when parsing with the [U] parameter present on thegoal symbol.

Syntax

Term[U, N]::[+U]Assertion[+U, ?N][+U]Atom[+U, ?N]Quantifier[+U]Atom[+U, ?N][~U]QuantifiableAssertion[?N]Quantifier[~U]Assertion[~U, ?N][~U]ExtendedAtom[?N]Quantifier[~U]ExtendedAtom[?N]Assertion[U, N]::^$\b\B[+U](?=Disjunction[+U, ?N])[+U](?!Disjunction[+U, ?N])[~U]QuantifiableAssertion[?N](?<=Disjunction[?U, ?N])(?<!Disjunction[?U, ?N])QuantifiableAssertion[N]::(?=Disjunction[~U, ?N])(?!Disjunction[~U, ?N])ExtendedAtom[N]::.\AtomEscape[~U, ?N]\[lookahead =c]CharacterClass[~U](Disjunction[~U, ?N])(?:Disjunction[~U, ?N])InvalidBracedQuantifierExtendedPatternCharacterInvalidBracedQuantifier::{DecimalDigits[~Sep]}{DecimalDigits[~Sep],}{DecimalDigits[~Sep],DecimalDigits[~Sep]}ExtendedPatternCharacter::SourceCharacterbut not one of^$\.*+?()[|AtomEscape[U, N]::[+U]DecimalEscape[~U]DecimalEscapebut only if theCapturingGroupNumberofDecimalEscapeis ≤ NcapturingParensCharacterClassEscape[?U]CharacterEscape[?U, ?N][+N]kGroupName[?U]CharacterEscape[U, N]::ControlEscapecControlLetter0[lookahead ∉DecimalDigit]HexEscapeSequenceRegExpUnicodeEscapeSequence[?U][~U]LegacyOctalEscapeSequenceIdentityEscape[?U, ?N]IdentityEscape[U, N]::[+U]SyntaxCharacter[+U]/[~U]SourceCharacterIdentityEscape[?N]SourceCharacterIdentityEscape[N]::[~N]SourceCharacterbut notc[+N]SourceCharacterbut not one ofcorkClassAtomNoDash[U, N]::SourceCharacterbut not one of\or]or-\ClassEscape[?U, ?N]\[lookahead =c]ClassEscape[U, N]::b[+U]-[~U]cClassControlLetterCharacterClassEscape[?U]CharacterEscape[?U, ?N]ClassControlLetter::DecimalDigit_Note

When the same left hand sides occurs with both [+U] and [~U] guards it is to control the disambiguation priority.

B.1.4.1 Static Semantics: Early Errors

The semantics of22.2.1.1is extended as follows:

ExtendedAtom::InvalidBracedQuantifier
  • It is a Syntax Error if any source text matches this rule.

Additionally, the rules for the following productions are modified with the addition of thehighlightedtext:

NonemptyClassRanges::ClassAtom-ClassAtomClassRangesNonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRanges

B.1.4.2 Static Semantics: IsCharacterClass

The semantics of22.2.1.3is extended as follows:

ClassAtomNoDash::\[lookahead =c]
  1. Returnfalse.

B.1.4.3 Static Semantics: CharacterValue

The semantics of22.2.1.4is extended as follows:

ClassAtomNoDash::\[lookahead =c]
  1. Return the code point value of U+005C (REVERSE SOLIDUS).
ClassEscape::cClassControlLetter
  1. Let ch be the code point matched byClassControlLetter.
  2. Let i be ch's code point value.
  3. Return the remainder of dividing i by 32.
CharacterEscape::LegacyOctalEscapeSequence
  1. Return the MV ofLegacyOctalEscapeSequence(seeB.1.2).

B.1.4.4 Pattern Semantics

The semantics of22.2.2is extended as follows:

Within22.2.2.5reference to “Atom::(GroupSpecifierDisjunction)” are to be interpreted as meaning “Atom::(GroupSpecifierDisjunction)” or “ExtendedAtom::(Disjunction)”.

Term (22.2.2.5) includes the following additional evaluation rules:

The productionTerm::QuantifiableAssertionQuantifierevaluates the same as the productionTerm::AtomQuantifierbut withQuantifiableAssertionsubstituted forAtom.

The productionTerm::ExtendedAtomQuantifierevaluates the same as the productionTerm::AtomQuantifierbut withExtendedAtomsubstituted forAtom.

The productionTerm::ExtendedAtomevaluates the same as the productionTerm::Atombut withExtendedAtomsubstituted forAtom.

Assertion (22.2.2.6) includes the following additional evaluation rule:

The productionAssertion::QuantifiableAssertionevaluates as follows:

  1. EvaluateQuantifiableAssertionto obtain a Matcher m.
  2. Return m.

Assertion (22.2.2.6) evaluation rules for theAssertion::(?=Disjunction)andAssertion::(?!Disjunction)productions are also used for theQuantifiableAssertionproductions, but withQuantifiableAssertionsubstituted forAssertion.

Atom (22.2.2.8) evaluation rules for theAtomproductions except forAtom::PatternCharacterare also used for theExtendedAtomproductions, but withExtendedAtomsubstituted forAtom. The following evaluation rules, with parameter direction, are also added:

The productionExtendedAtom::\[lookahead =c]evaluates as follows:

  1. Let A be the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
  2. Return ! CharacterSetMatcher(A,false, direction).

The productionExtendedAtom::ExtendedPatternCharacterevaluates as follows:

  1. Let ch be the character represented byExtendedPatternCharacter.
  2. Let A be a one-element CharSet containing the character ch.
  3. Return ! CharacterSetMatcher(A,false, direction).

CharacterEscape (22.2.2.10) includes the following additional evaluation rule:

The productionCharacterEscape::LegacyOctalEscapeSequenceevaluates as follows:

  1. Let cv be theCharacterValueof thisCharacterEscape.
  2. Return the character whose character value is cv.

NonemptyClassRanges (22.2.2.15) modifies the following evaluation rule:

The productionNonemptyClassRanges::ClassAtom-ClassAtomClassRangesevaluates as follows:

  1. Evaluate the firstClassAtomto obtain a CharSet A.
  2. Evaluate the secondClassAtomto obtain a CharSet B.
  3. EvaluateClassRangesto obtain a CharSet C.
  4. Let D be ! CharacterRangeOrUnion(A, B).
  5. Return the union of D and C.

NonemptyClassRangesNoDash (22.2.2.16) modifies the following evaluation rule:

The productionNonemptyClassRangesNoDash::ClassAtomNoDash-ClassAtomClassRangesevaluates as follows:

  1. EvaluateClassAtomNoDashto obtain a CharSet A.
  2. EvaluateClassAtomto obtain a CharSet B.
  3. EvaluateClassRangesto obtain a CharSet C.
  4. Let D be ! CharacterRangeOrUnion(A, B).
  5. Return the union of D and C.

ClassEscape (22.2.2.19) includes the following additional evaluation rule:

The productionClassEscape::cClassControlLetterevaluates as follows:

  1. Let cv be theCharacterValueof thisClassEscape.
  2. Let c be the character whose character value is cv.
  3. Return the CharSet containing the single character c.

ClassAtomNoDash (22.2.2.18) includes the following additional evaluation rule:

The productionClassAtomNoDash::\[lookahead =c]evaluates as follows:

  1. Return the CharSet containing the single character \ U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence \c within a character class where it is not followed by an acceptable control character.

B.1.4.4.1 CharacterRangeOrUnion ( A, B )

The abstract operation CharacterRangeOrUnion takes arguments A (a CharSet) and B (a CharSet). It performs the following steps when called:

  1. If Unicode isfalse, then
    1. If A does not contain exactly one character or B does not contain exactly one character, then
      1. Let C be the CharSet containing the single character - U+002D (HYPHEN-MINUS).
      2. Return the union of CharSets A, B and C.
  2. Return ! CharacterRange(A, B).

B.2 Additional Built-in Properties

When the ECMAScripthostis a web browser the following additional properties of the standard built-in objects are defined.

B.2.1 Additional Properties of the Global Object

The entries inTable 85are added toTable 8.

Table 85: Additional Well-known Intrinsic Objects
Intrinsic NameGlobal NameECMAScript Language Association
%escape%escapeThe escape function (B.2.1.1)
%unescape%unescapeThe unescape function (B.2.1.2)

B.2.1.1 escape ( string )

The escape function is a property of theglobal object. It computes a new version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.

For those code units being replaced whose value is 0x00FF or less, a two-digit escape sequence of the form %xx is used. For those characters being replaced whose code unit value is greater than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.

The escape function is the %escape% intrinsic object. When the escape function is called with one argument string, the following steps are taken:

  1. Set string to ? ToString(string).
  2. Let length be the number of code units in string.
  3. Let R be the empty String.
  4. Let k be 0.
  5. Repeat, while k < length,
    1. Let char be the code unit (represented as a 16-bit unsignedinteger) at index k within string.
    2. If char is one of the code units in"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./", then
      1. Let S be the String value containing the single code unit char.
    3. Else if char ≥ 256, then
      1. Let n be the numeric value of char.
      2. Let S be thestring-concatenationof:
        • "%u"
        • the String representation of n, formatted as a four-digit uppercase hexadecimal number, padded to the left with zeroes if necessary
    4. Else,
      1. Assert: char < 256.
      2. Let n be the numeric value of char.
      3. Let S be thestring-concatenationof:
        • "%"
        • the String representation of n, formatted as a two-digit uppercase hexadecimal number, padded to the left with a zero if necessary
    5. Set R to thestring-concatenationof R and S.
    6. Set k to k + 1.
  6. Return R.
Note

The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in this standard is described above without regard to the contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC 3986.

B.2.1.2 unescape ( string )

The unescape function is a property of theglobal object. It computes a new version of a String value in which each escape sequence of the sort that might be introduced by the escape function is replaced with the code unit that it represents.

The unescape function is the %unescape% intrinsic object. When the unescape function is called with one argument string, the following steps are taken:

  1. Set string to ? ToString(string).
  2. Let length be the number of code units in string.
  3. Let R be the empty String.
  4. Let k be 0.
  5. Repeat, while klength,
    1. Let c be the code unit at index k within string.
    2. If c is the code unit 0x0025 (PERCENT SIGN), then
      1. Let hexEscape be the empty String.
      2. Let skip be 0.
      3. If klength - 6 and the code unit at index k + 1 within string is the code unit 0x0075 (LATIN SMALL LETTER U), then
        1. Set hexEscape to thesubstringof string from k + 2 to k + 6.
        2. Set skip to 5.
      4. Else if klength - 3, then
        1. Set hexEscape to thesubstringof string from k + 1 to k + 3.
        2. Set skip to 2.
      5. If hexEscape can be interpreted as an expansion ofHexDigits[~Sep], then
        1. Let hexIntegerLiteral be thestring-concatenationof"0x"and hexEscape.
        2. Let n be ! ToNumber(hexIntegerLiteral).
        3. Set c to the code unit whose value is(n).
        4. Set k to k + skip.
    3. Set R to thestring-concatenationof R and c.
    4. Set k to k + 1.
  6. Return R.

B.2.2 Additional Properties of the String.prototype Object

B.2.2.1 String.prototype.substr ( start, length )

The substr method takes two arguments, start and length, and returns asubstringof the result of converting thethisvalue to a String, starting from index start and running for length code units (or through the end of the String if length isundefined). If start is negative, it is treated assourceLength + startwhere sourceLength is the length of the String. The result is a String value, not a String object. The following steps are taken:

  1. Let O be ? RequireObjectCoercible(thisvalue).
  2. Let S be ? ToString(O).
  3. Let size be the length of S.
  4. Let intStart be ? ToIntegerOrInfinity(start).
  5. If intStart is -∞, set intStart to 0.
  6. Else if intStart < 0, set intStart tomax(size + intStart, 0).
  7. If length isundefined, let intLength be size; otherwise let intLength be ? ToIntegerOrInfinity(length).
  8. If intStart is +∞, intLength ≤ 0, or intLength is +∞, return the empty String.
  9. Let intEnd bemin(intStart + intLength, size).
  10. If intStartintEnd, return the empty String.
  11. Return thesubstringof S from intStart to intEnd.
Note

The substr function is intentionally generic; it does not require that itsthisvalue be a String object. Therefore it can be transferred to other kinds of objects for use as a method.

B.2.2.2 String.prototype.anchor ( name )

When the anchor method is called with argument name, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"a","name", name).

B.2.2.2.1 CreateHTML ( string, tag, attribute, value )

The abstract operation CreateHTML takes arguments string, tag (a String), attribute (a String), and value. It performs the following steps when called:

  1. Let str be ? RequireObjectCoercible(string).
  2. Let S be ? ToString(str).
  3. Let p1 be thestring-concatenationof"<"and tag.
  4. If attribute is not the empty String, then
    1. Let V be ? ToString(value).
    2. Let escapedV be the String value that is the same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six code unit sequence"&quot;".
    3. Set p1 to thestring-concatenationof:
      • p1
      • the code unit 0x0020 (SPACE)
      • attribute
      • the code unit 0x003D (EQUALS SIGN)
      • the code unit 0x0022 (QUOTATION MARK)
      • escapedV
      • the code unit 0x0022 (QUOTATION MARK)
  5. Let p2 be thestring-concatenationof p1 and">".
  6. Let p3 be thestring-concatenationof p2 and S.
  7. Let p4 be thestring-concatenationof p3,"</", tag, and">".
  8. Return p4.

B.2.2.3 String.prototype.big ( )

When the big method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"big","","").

B.2.2.4 String.prototype.blink ( )

When the blink method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"blink","","").

B.2.2.5 String.prototype.bold ( )

When the bold method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"b","","").

B.2.2.6 String.prototype.fixed ( )

When the fixed method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"tt","","").

B.2.2.7 String.prototype.fontcolor ( color )

When the fontcolor method is called with argument color, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"font","color", color).

B.2.2.8 String.prototype.fontsize ( size )

When the fontsize method is called with argument size, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"font","size", size).

B.2.2.9 String.prototype.italics ( )

When the italics method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"i","","").

B.2.2.10 String.prototype.link ( url )

When the link method is called with argument url, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"a","href", url).

B.2.2.11 String.prototype.small ( )

When the small method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"small","","").

B.2.2.12 String.prototype.strike ( )

When the strike method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"strike","","").

B.2.2.13 String.prototype.sub ( )

When the sub method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"sub","","").

B.2.2.14 String.prototype.sup ( )

When the sup method is called with no arguments, the following steps are taken:

  1. Let S be thethisvalue.
  2. Return ? CreateHTML(S,"sup","","").

B.2.2.15 String.prototype.trimLeft ( )

Note

The property"trimStart"is preferred. The"trimLeft"property is provided principally for compatibility with old code. It is recommended that the"trimStart"property be used in new ECMAScript code.

The initial value of the"trimLeft"property is the samefunction objectas the initial value of the String.prototype.trimStart property.

B.2.2.16 String.prototype.trimRight ( )

Note

The property"trimEnd"is preferred. The"trimRight"property is provided principally for compatibility with old code. It is recommended that the"trimEnd"property be used in new ECMAScript code.

The initial value of the"trimRight"property is the samefunction objectas the initial value of the String.prototype.trimEnd property.

B.2.3 Additional Properties of the Date.prototype Object

B.2.3.1 Date.prototype.getYear ( )

Note

The getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the getYear method is called with no arguments, the following steps are taken:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, returnNaN.
  3. ReturnYearFromTime(LocalTime(t)) -1900𝔽.

B.2.3.2 Date.prototype.setYear ( year )

Note

The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the setYear method is called with one argument year, the following steps are taken:

  1. Let t be ? thisTimeValue(thisvalue).
  2. If t isNaN, set t to+0𝔽; otherwise, set t toLocalTime(t).
  3. Let y be ? ToNumber(year).
  4. If y isNaN, then
    1. Set the [[DateValue]] internal slot ofthis Date objecttoNaN.
    2. ReturnNaN.
  5. Let yi be ! ToIntegerOrInfinity(y).
  6. If 0 ≤ yi ≤ 99, let yyyy be1900𝔽 +𝔽(yi).
  7. Else, let yyyy be y.
  8. Let d beMakeDay(yyyy,MonthFromTime(t),DateFromTime(t)).
  9. Let date beUTC(MakeDate(d,TimeWithinDay(t))).
  10. Set the [[DateValue]] internal slot ofthis Date objecttoTimeClip(date).
  11. Return the value of the [[DateValue]] internal slot ofthis Date object.

B.2.3.3 Date.prototype.toGMTString ( )

Note

The toUTCString method is preferred. The toGMTString method is provided principally for compatibility with old code.

Thefunction objectthat is the initial value of Date.prototype.toGMTString is the samefunction objectthat is the initial value of Date.prototype.toUTCString.

B.2.4 Additional Properties of the RegExp.prototype Object

B.2.4.1 RegExp.prototype.compile ( pattern, flags )

When the compile method is called with arguments pattern and flags, the following steps are taken:

  1. Let O be thethisvalue.
  2. Perform ? RequireInternalSlot(O, [[RegExpMatcher]]).
  3. IfType(pattern) is Object and pattern has a [[RegExpMatcher]] internal slot, then
    1. If flags is notundefined, throw aTypeErrorexception.
    2. Let P be pattern.[[OriginalSource]].
    3. Let F be pattern.[[OriginalFlags]].
  4. Else,
    1. Let P be pattern.
    2. Let F be flags.
  5. Return ? RegExpInitialize(O, P, F).
Note

The compile method completely reinitializes thethisvalue RegExp with a new pattern and flags. An implementation may interpret use of this method as an assertion that the resulting RegExp object will be used multiple times and hence is a candidate for extra optimization.

B.3 Other Additional Features

B.3.1 Labelled Function Declarations

Prior to ECMAScript 2015, the specification ofLabelledStatementdid not allow for the association of a statement label with aFunctionDeclaration. However, a labelledFunctionDeclarationwas an allowable extension fornon-strict codeand most browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar production forLabelledStatementpermits use ofFunctionDeclarationas aLabelledItembut14.13.1includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the addition of thehighlightedtext:

LabelledItem:FunctionDeclaration
  • It is a Syntax Error if anystrict modesource code matches this rule.
Note

Theearly errorrules forWithStatement,IfStatement, andIterationStatementprevent these statements from containing a labelledFunctionDeclarationinnon-strict code.

B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics

Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of aFunctionDeclarationas an element of aBlockstatement'sStatementList. However, support for that form ofFunctionDeclarationwas an allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the semantics of such declarations differ among those implementations. Because of these semantic differences, existing web ECMAScript code that usesBlocklevel function declarations is only portable among browser implementation if the usage only depends upon the semantic intersection of all of the browser implementations for such declarations. The following are the use cases that fall within that intersection semantics:

  1. A function is declared and only referenced within a single block

  2. A function is declared and possibly used within a singleBlockbut also referenced by an inner function definition that is not contained within that sameBlock.

    • One or moreFunctionDeclarations whoseBindingIdentifieris the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock.
    • No other declaration of f that is not a var declaration occurs within the function code of g
    • There may be occurrences of f as anIdentifierReferencewithin theStatementListof theBlockcontaining the declaration of f.
    • There is at least one occurrence of f as anIdentifierReferencewithin another function h that is nested within g and no other declaration of f shadows the references to f from within h.
    • All invocations of h occur after the declaration of f has been evaluated.
  3. A function is declared and possibly used within a single block but also referenced within subsequent blocks.

    • One or moreFunctionDeclarationwhoseBindingIdentifieris the name f occur within the function code of an enclosing function g and that declaration is nested within aBlock.
    • No other declaration of f that is not a var declaration occurs within the function code of g
    • There may be occurrences of f as anIdentifierReferencewithin theStatementListof theBlockcontaining the declaration of f.
    • There is at least one occurrence of f as anIdentifierReferencewithin the function code of g that lexically follows theBlockcontaining the declaration of f.

The first use case is interoperable with the semantics ofBlocklevel function declarations provided by ECMAScript 2015. Any pre-existing ECMAScript code that employs that use case will operate using the Block level function declarations semantics defined by clauses10,14, and15.

ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to the clause10, clause15, clause19.2.1and clause16.1.7semantics.

If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should be produced when code contains aFunctionDeclarationfor which these compatibility semantics are applied and introduce observable differences from non-compatibility semantics. For example, if a var binding is not introduced because its introduction would create anearly error, a warning message should not be produced.

B.3.2.1 Changes to FunctionDeclarationInstantiation

DuringFunctionDeclarationInstantiationthe following steps are performed in place of step29:

  1. If strict isfalse, then
    1. For eachFunctionDeclarationf that is directly contained in theStatementListof aBlock,CaseClause, orDefaultClause, do
      1. Let F beStringValueof theBindingIdentifierof f.
      2. If replacing theFunctionDeclarationf with aVariableStatementthat has F as aBindingIdentifierwould not produce any Early Errors for func and F is not an element of parameterNames, then
        1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or anotherFunctionDeclaration.
        2. If initializedBindings does not contain F and F is not"arguments", then
          1. Perform ! varEnv.CreateMutableBinding(F,false).
          2. Perform varEnv.InitializeBinding(F,undefined).
          3. Append F to instantiatedVarNames.
        3. When theFunctionDeclarationf is evaluated, perform the following steps in place of theFunctionDeclarationEvaluation algorithm provided in15.2.6:
          1. Let fenv be therunning execution context's VariableEnvironment.
          2. Let benv be therunning execution context's LexicalEnvironment.
          3. Let fobj be ! benv.GetBindingValue(F,false).
          4. Perform ! fenv.SetMutableBinding(F, fobj,false).
          5. ReturnNormalCompletion(empty).

B.3.2.2 Changes to GlobalDeclarationInstantiation

DuringGlobalDeclarationInstantiationthe following steps are performed in place of step13:

  1. Let strict beIsStrictof script.
  2. If strict isfalse, then
    1. Let declaredFunctionOrVarNames be thelist-concatenationof declaredFunctionNames and declaredVarNames.
    2. For eachFunctionDeclarationf that is directly contained in theStatementListof aBlock,CaseClause, orDefaultClauseContained within script, do
      1. Let F beStringValueof theBindingIdentifierof f.
      2. If replacing theFunctionDeclarationf with aVariableStatementthat has F as aBindingIdentifierwould not produce any Early Errors for script, then
        1. If env.HasLexicalDeclaration(F) isfalse, then
          1. Let fnDefinable be ? env.CanDeclareGlobalVar(F).
          2. If fnDefinable istrue, then
            1. NOTE: A var binding for F is only instantiated here if it is neither a VarDeclaredName nor the name of anotherFunctionDeclaration.
            2. If declaredFunctionOrVarNames does not contain F, then
              1. Perform ? env.CreateGlobalVarBinding(F,false).
              2. Append F to declaredFunctionOrVarNames.
            3. When theFunctionDeclarationf is evaluated, perform the following steps in place of theFunctionDeclarationEvaluation algorithm provided in15.2.6:
              1. Let genv be therunning execution context's VariableEnvironment.
              2. Let benv be therunning execution context's LexicalEnvironment.
              3. Let fobj be ! benv.GetBindingValue(F,false).
              4. Perform ? genv.SetMutableBinding(F, fobj,false).
              5. ReturnNormalCompletion(empty).

B.3.2.3 Changes to EvalDeclarationInstantiation

DuringEvalDeclarationInstantiationthe following steps are performed in place of step11:

  1. If strict isfalse, then
    1. Let declaredFunctionOrVarNames be thelist-concatenationof declaredFunctionNames and declaredVarNames.
    2. For eachFunctionDeclarationf that is directly contained in theStatementListof aBlock,CaseClause, orDefaultClauseContained within body, do
      1. Let F beStringValueof theBindingIdentifierof f.
      2. If replacing theFunctionDeclarationf with aVariableStatementthat has F as aBindingIdentifierwould not produce any Early Errors for body, then
        1. Let bindingExists befalse.
        2. Let thisEnv be lexEnv.
        3. Assert: The following loop will terminate.
        4. Repeat, while thisEnv is not the same as varEnv,
          1. If thisEnv is not anobject Environment Record, then
            1. If thisEnv.HasBinding(F) istrue, then
              1. Let bindingExists betrue.
          2. Set thisEnv to thisEnv.[[OuterEnv]].
        5. If bindingExists isfalseand varEnv is aglobal Environment Record, then
          1. If varEnv.HasLexicalDeclaration(F) isfalse, then
            1. Let fnDefinable be ? varEnv.CanDeclareGlobalVar(F).
          2. Else,
            1. Let fnDefinable befalse.
        6. Else,
          1. Let fnDefinable betrue.
        7. If bindingExists isfalseand fnDefinable istrue, then
          1. If declaredFunctionOrVarNames does not contain F, then
            1. If varEnv is aglobal Environment Record, then
              1. Perform ? varEnv.CreateGlobalVarBinding(F,true).
            2. Else,
              1. Let bindingExists be varEnv.HasBinding(F).
              2. If bindingExists isfalse, then
                1. Perform ! varEnv.CreateMutableBinding(F,true).
                2. Perform ! varEnv.InitializeBinding(F,undefined).
            3. Append F to declaredFunctionOrVarNames.
          2. When theFunctionDeclarationf is evaluated, perform the following steps in place of theFunctionDeclarationEvaluation algorithm provided in15.2.6:
            1. Let genv be therunning execution context's VariableEnvironment.
            2. Let benv be therunning execution context's LexicalEnvironment.
            3. Let fobj be ! benv.GetBindingValue(F,false).
            4. Perform ? genv.SetMutableBinding(F, fobj,false).
            5. ReturnNormalCompletion(empty).

B.3.2.4 Changes to Block Static Semantics: Early Errors

The rules for the following production in14.2.1are modified with the addition of thehighlightedtext:

Block:{StatementList}

B.3.2.5 Changes to switch Statement Static Semantics: Early Errors

The rules for the following production in14.12.1are modified with the addition of thehighlightedtext:

SwitchStatement:switch(Expression)CaseBlock

B.3.2.6 Changes to BlockDeclarationInstantiation

DuringBlockDeclarationInstantiationthe following steps are performed in place of step4.a.ii.1:

  1. If env.HasBinding(dn) isfalse, then
    1. Perform ! env.CreateMutableBinding(dn,false).

DuringBlockDeclarationInstantiationthe following steps are performed in place of step4.b.iii:

  1. If the binding for fn in env is an uninitialized binding, then
    1. Perform env.InitializeBinding(fn, fo).
  2. Else,
    1. Assert: d is aFunctionDeclaration.
    2. Perform env.SetMutableBinding(fn, fo,false).

B.3.3 FunctionDeclarations in IfStatement Statement Clauses

The following augments theIfStatementproduction in14.6:

IfStatement[Yield, Await, Return]:if(Expression[+In, ?Yield, ?Await])FunctionDeclaration[?Yield, ?Await, ~Default]elseStatement[?Yield, ?Await, ?Return]if(Expression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]elseFunctionDeclaration[?Yield, ?Await, ~Default]if(Expression[+In, ?Yield, ?Await])FunctionDeclaration[?Yield, ?Await, ~Default]elseFunctionDeclaration[?Yield, ?Await, ~Default]if(Expression[+In, ?Yield, ?Await])FunctionDeclaration[?Yield, ?Await, ~Default][lookahead ≠else]

This production only applies when parsingnon-strict code. Code matching this production is processed as if each matching occurrence ofFunctionDeclaration[?Yield, ?Await, ~Default]was the soleStatementListItemof aBlockStatementoccupying that position in the source code. The semantics of such a syntheticBlockStatementincludes the web legacy compatibility semantics specified inB.3.2.

B.3.4 VariableStatements in Catch Blocks

The content of subclause14.15.1is replaced with the following:

Catch:catch(CatchParameter)BlockNote

TheBlockof aCatchclause may contain var declarations that bind a name that is also bound by theCatchParameter. At runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named bindings introduced by theCatchParameterand hence theInitializerfor such var declarations will assign to the corresponding catch parameter rather than the var binding.

This modified behaviour also applies to var and function declarations introduced bydirect evalcalls contained within theBlockof aCatchclause. This change is accomplished by modifying the algorithm of19.2.1.3as follows:

Step3.d.i.2.a.iis replaced by:

  1. If thisEnv is not theEnvironment Recordfor aCatchclause, throw aSyntaxErrorexception.

Step11.b.ii.4.a.i.iis replaced by:

  1. If thisEnv is not theEnvironment Recordfor aCatchclause, let bindingExists betrue.

B.3.5 Initializers in ForIn Statement Heads

The following augments theForInOfStatementproduction in14.7.5:

ForInOfStatement[Yield, Await, Return]:for(varBindingIdentifier[?Yield, ?Await]Initializer[~In, ?Yield, ?Await]inExpression[+In, ?Yield, ?Await])Statement[?Yield, ?Await, ?Return]

This production only applies when parsingnon-strict code.

Thestatic semanticsofContainsDuplicateLabelsin8.2.1are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. ReturnContainsDuplicateLabelsofStatementwith argument labelSet.

Thestatic semanticsofContainsUndefinedBreakTargetin8.2.2are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. ReturnContainsUndefinedBreakTargetofStatementwith argument labelSet.

Thestatic semanticsofContainsUndefinedContinueTargetin8.2.3are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. ReturnContainsUndefinedContinueTargetofStatementwith arguments iterationSet and « ».

Thestatic semanticsofIsDestructuringin14.7.5.2are augmented with the following:

BindingIdentifier:Identifieryieldawait
  1. Returnfalse.

Thestatic semanticsofVarDeclaredNamesin8.1.6are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. Let names1 be theBoundNamesofBindingIdentifier.
  2. Let names2 be theVarDeclaredNamesofStatement.
  3. Return thelist-concatenationof names1 and names2.

Thestatic semanticsofVarScopedDeclarationsin8.1.7are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. Let declarations1 be aListwhose sole element isBindingIdentifier.
  2. Let declarations2 be theVarScopedDeclarationsofStatement.
  3. Return thelist-concatenationof declarations1 and declarations2.

Theruntime semanticsofForInOfLoopEvaluationin14.7.5.5are augmented with the following:

ForInOfStatement:for(varBindingIdentifierInitializerinExpression)Statement
  1. Let bindingId beStringValueofBindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. IfIsAnonymousFunctionDefinition(Initializer) istrue, then
    1. Let value beNamedEvaluationofInitializerwith argument bindingId.
  4. Else,
    1. Let rhs be the result of evaluatingInitializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? PutValue(lhs, value).
  6. Let keyResult be ?ForIn/OfHeadEvaluation(« »,Expression,enumerate).
  7. Return ?ForIn/OfBodyEvaluation(BindingIdentifier,Statement, keyResult,enumerate,varBinding, labelSet).

B.3.6 The [[IsHTMLDDA]] Internal Slot

An [[IsHTMLDDA]] internal slot may exist onhost-definedobjects. Objects with an [[IsHTMLDDA]] internal slot behave likeundefinedin theToBooleanandIsLooselyEqualabstract operationsand when used as an operand for thetypeof operator.

Note

Objects with an [[IsHTMLDDA]] internal slot are never created by this specification. However, the document.all object in web browsers is ahost-definedexotic objectwith this slot that exists for web compatibility purposes. There are no other known examples of this type of object and implementations should not create any with the exception of document.all.

B.3.6.1 Changes to ToBoolean

The result column inTable 13for an argument type of Object is replaced with the following algorithm:

  1. If argument has an[[IsHTMLDDA]] internal slot, returnfalse.
  2. Returntrue.

B.3.6.2 Changes to IsLooselyEqual

The following steps replace step4of theIsLooselyEqualalgorithm:

  1. IfType(x) is Object and x has an[[IsHTMLDDA]] internal slotand y is eithernullorundefined, returntrue.
  2. If x is eithernullorundefinedandType(y) is Object and y has an[[IsHTMLDDA]] internal slot, returntrue.

B.3.6.3 Changes to the typeof Operator

The following table entry is inserted intoTable 41immediately preceding the entry for "Object (implements [[Call]])":

Table 86: AdditionaltypeofOperator Results
Type of valResult
Object (has an[[IsHTMLDDA]] internal slot)"undefined"

C The Strict Mode of ECMAScript

The strict mode restriction and exceptions

D Host Layering Points

See4.2for the definition ofhost.

D.1 Host Hooks

HostCallJobCallback(...)

HostEnqueueFinalizationRegistryCleanupJob(...)

HostEnqueuePromiseJob(...)

HostEnsureCanCompileStrings(...)

HostFinalizeImportMeta(...)

HostGetImportMetaProperties(...)

HostHasSourceTextAvailable(...)

HostImportModuleDynamically(...)

HostMakeJobCallback(...)

HostPromiseRejectionTracker(...)

HostResolveImportedModule(...)

InitializeHostDefinedRealm(...)

D.2 Host-defined Fields

[[HostDefined]] onRealmRecords: SeeTable 27.

[[HostDefined]] on Script Records: SeeTable 43.

[[HostDefined]] on Module Records: SeeTable 44.

[[HostDefined]] on JobCallback Records: SeeTable 31.

[[HostSynchronizesWith]] on Candidate Executions: SeeTable 84.

[[IsHTMLDDA]]: SeeB.3.6.

D.3 Host-defined Objects

Theglobal object: See clause19.

D.4 Running Jobs

Preparation steps before, and cleanup steps after, invocation ofJobAbstract Closures. See9.5.

D.5 Internal Methods of Exotic Objects

Any of the essential internal methods inTable 6for anyexotic objectnot specified within this specification.

D.6 Built-in Objects and Methods

Any built-in objects and methods not defined within this specification, except as restricted in17.1.

E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility Impact

9.1.1.4.15-9.1.1.4.18Edition 5 and 5.1 used a property existence test to determine whether aglobal objectproperty corresponding to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to what has been most commonly implemented by web browsers.

10.4.2.1: The 5th Edition moved the capture of the current array length prior to theintegerconversion of thearray indexor new length value. However, the captured length value could become invalid if the conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must be captured after the possible occurrence of such side-effects.

21.4.1.14: Previous editions permitted theTimeClipabstract operation to return either+0𝔽 or-0𝔽 as the representation of a 0time value. ECMAScript 2015 specifies that+0𝔽 always returned. This means that for ECMAScript 2015 thetime valueof a Date object is never observably-0𝔽 and methods that return time values never return-0𝔽.

21.4.1.15: If a UTC offset representation is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should be interpreted as"z".

21.4.4.36: If the year cannot be represented using the Date Time String Format specified in21.4.1.15a RangeError exception is thrown. Previous editions did not specify the behaviour for that case.

21.4.4.41: Previous editions did not specify the value returned by Date.prototype.toString whenthis time valueisNaN. ECMAScript 2015 specifies the result to be the String value"Invalid Date".

22.2.3.1,22.2.3.2.5: Any LineTerminator code points in the value of the"source"property of a RegExp instance must be expressed using an escape sequence. Edition 5.1 only required the escaping of /.

22.2.5.7,22.2.5.10: In previous editions, the specifications for String.prototype.match and String.prototype.replace was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The previous specifications stated that for each attempt to match the pattern, if lastIndex did not change it should be incremented by 1. The correct behaviour is that lastIndex should be incremented by one only if the pattern matched the empty String.

23.1.3.27,23.1.3.27.1: Previous editions did not specify how aNaNvalue returned by a comparefn was interpreted by Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if+0𝔽 was returned from the comparefn. ECMAScript 2015 also specifies thatToNumberis applied to the result returned by a comparefn. In previous editions, the effect of a comparefn result that is not aNumber valuewasimplementation-defined. In practice, implementations callToNumber.

F Additions and Changes That Introduce Incompatibilities with Prior Editions

6.2.4: In ECMAScript 2015, Function calls are not allowed to return aReference Record.

7.1.4.1: In ECMAScript 2015,ToNumberapplied to a String value now recognizes and convertsBinaryIntegerLiteralandOctalIntegerLiteralnumeric strings. In previous editions such strings were converted toNaN.

9.3: In ECMAScript 2018, Template objects are canonicalized based onParse Node(source location), instead of across all occurrences of that template literal or tagged template in aRealmin previous editions.

12.2: In ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example, "\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode standard.

12.6: In ECMAScript 2015, the valid code points for anIdentifierNameare specified in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the validIdentifierNameorIdentifiercode points were specified by enumerating various Unicode code point categories.

12.9.1: In ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is missing. This change aligns the specification with the actual behaviour of most existing implementations.

13.2.5.1: In ECMAScript 2015, it is no longer anearly errorto have duplicate property names in Object Initializers.

13.15.1: In ECMAScript 2015,strict mode codecontaining an assignment to an immutable binding such as the function name of aFunctionExpressiondoes not produce anearly error. Instead it produces a runtime error.

14.2: In ECMAScript 2015, aStatementListbeginning with the token let followed by the input elementsLineTerminatorthenIdentifieris the start of aLexicalDeclaration. In previous editions, automatic semicolon insertion would always insert a semicolon before theIdentifierinput element.

14.5: In ECMAScript 2015, aStatementListItembeginning with the token let followed by the token [ is the start of aLexicalDeclaration. In previous editions such a sequence would be the start of anExpressionStatement.

14.6.2: In ECMAScript 2015, thenormal completionvalue of anIfStatementis never the valueempty. If noStatementpart is evaluated or if the evaluatedStatementpart produces anormal completionwhose value isempty, the completion value of theIfStatementisundefined.

14.7: In ECMAScript 2015, if the ( token of a for statement is immediately followed by the token sequence let [ then the let is treated as the start of aLexicalDeclaration. In previous editions such a token sequence would be the start of anExpression.

14.7: In ECMAScript 2015, if the ( token of a for-in statement is immediately followed by the token sequence let [ then the let is treated as the start of aForDeclaration. In previous editions such a token sequence would be the start of anLeftHandSideExpression.

14.7: Prior to ECMAScript 2015, an initialization expression could appear as part of theVariableDeclarationthat precedes the inkeyword. In ECMAScript 2015, theForBindingin that same position does not allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only innon-strict code.

14.7: In ECMAScript 2015, the completion value of anIterationStatementis never the valueempty. If theStatementpart of anIterationStatementis not evaluated or if the final evaluation of theStatementpart produces a completion whose value isempty, the completion value of theIterationStatementisundefined.

14.11.2: In ECMAScript 2015, thenormal completionvalue of aWithStatementis never the valueempty. If evaluation of theStatementpart of aWithStatementproduces anormal completionwhose value isempty, the completion value of theWithStatementisundefined.

14.12.4: In ECMAScript 2015, the completion value of aSwitchStatementis never the valueempty. If theCaseBlockpart of aSwitchStatementproduces a completion whose value isempty, the completion value of theSwitchStatementisundefined.

14.15: In ECMAScript 2015, it is anearly errorfor aCatchclause to contain a var declaration for the sameIdentifierthat appears as theCatchclause parameter. In previous editions, such a variable declaration would be instantiated in the enclosing variable environment but the declaration'sInitializervalue would be assigned to theCatchparameter.

14.15,19.2.1.3: In ECMAScript 2015, a runtimeSyntaxErroris thrown if aCatchclause evaluates a non-strict direct eval whose eval code includes a var or FunctionDeclaration declaration that binds the sameIdentifierthat appears as theCatchclause parameter.

14.15.3: In ECMAScript 2015, the completion value of aTryStatementis never the valueempty. If theBlockpart of aTryStatementevaluates to anormal completionwhose value isempty, the completion value of theTryStatementisundefined. If theBlockpart of aTryStatementevaluates to athrow completionand it has aCatchpart that evaluates to anormal completionwhose value isempty, the completion value of theTryStatementisundefinedif there is noFinallyclause or if itsFinallyclause evaluates to anemptynormal completion.

15.4.5In ECMAScript 2015, the function objects that are created as the values of the [[Get]] or [[Set]] attribute of accessor properties in anObjectLiteralare notconstructorfunctions and they do not have a"prototype"own property. In the previous edition, they were constructors and had a"prototype"property.

20.1.2.6: In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.8: In ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.10: In ECMAScript 2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.12: In ECMAScript 2015, if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.14: In ECMAScript 2015, if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.15: In ECMAScript 2015, if the argument to Object.isFrozen is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.16: In ECMAScript 2015, if the argument to Object.isSealed is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.17: In ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to coerce the argument usingToObject. If the coercion is successful the result is used in place of the original argument value. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.18: In ECMAScript 2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.1.2.20: In ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was a non-extensibleordinary objectwith no own properties. In the previous edition, a non-object argument always causes aTypeErrorto be thrown.

20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to%Function.prototype%.

20.2.4.1: In ECMAScript 2015, the"length"property of function instances is configurable. In previous editions it was non-configurable.

20.5.6.2: In ECMAScript 2015, the [[Prototype]] internal slot of a NativeErrorconstructoris the Errorconstructor. In previous editions it was theFunction prototype object.

21.4.4In ECMAScript 2015, theDate prototype objectis not a Date instance. In previous editions it was a Date instance whose TimeValue wasNaN.

22.1.3.10In ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are canonically equivalent according to the Unicode standard as being identical. In previous editions implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.

22.1.3.26and22.1.3.28In ECMAScript 2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion processing was only applied to individual code units. The only affected code points are those in the Deseret block of Unicode.

22.1.3.29In ECMAScript 2015, the String.prototype.trim method is defined to recognize white space code points that may exist outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such code points would not have been recognized as white space.

22.2.3.1In ECMAScript 2015, If the pattern argument is a RegExp instance and the flags argument is notundefined, a new RegExp instance is created just like pattern except that pattern's flags are replaced by the argument flags. In previous editions aTypeErrorexception was thrown when pattern was a RegExp instance and flags was notundefined.

22.2.5In ECMAScript 2015, theRegExp prototype objectis not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty String.

22.2.5In ECMAScript 2015,"source","global","ignoreCase", and"multiline"are accessor properties defined on theRegExp prototype object. In previous editions they were data properties defined on RegExp instances.

25.4.13: In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent confusion with Atomics.wait.

27.1.4.4,27.6.3.6: In ECMAScript 2019, the number of Jobs enqueued by await was reduced, which could create an observable difference in resolution order between a then() call and an await expression.

G Colophon

This specification is authored on GitHub in a plaintext source format called Ecmarkup. Ecmarkup is an HTML and Markdown dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and processing the specification into a full-featured HTML rendering that follows the editorial conventions for this document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of this specification are produced by printing the HTML rendering to a PDF.

Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated conversion tool.

H Bibliography

  1. IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of Electrical and Electronic Engineers, New York (2019)Note

    There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262 specification.

  2. The Unicode Standard, available at <https://unicode.org/versions/latest>
  3. Unicode Technical Note #5: Canonical Equivalence in Applications, available at <https://unicode.org/notes/tn5/>
  4. Unicode Technical Standard #10: Unicode Collation Algorithm, available at <https://unicode.org/reports/tr10/>
  5. Unicode Standard Annex #15, Unicode Normalization Forms, available at <https://unicode.org/reports/tr15/>
  6. Unicode Standard Annex #18: Unicode Regular Expressions, available at <https://unicode.org/reports/tr18/>
  7. Unicode Standard Annex #24: Unicode Script Property, available at <https://unicode.org/reports/tr24/>
  8. Unicode Standard Annex #31, Unicode Identifiers and Pattern Syntax, available at <https://unicode.org/reports/tr31/>
  9. Unicode Standard Annex #44: Unicode Character Database, available at <https://unicode.org/reports/tr44/>
  10. Unicode Technical Standard #51: Unicode Emoji, available at <https://unicode.org/reports/tr51/>
  11. IANA Time Zone Database, available at <https://www.iana.org/time-zones>
  12. ISO 8601:2004(E) Data elements and interchange formats — Information interchange — Representation of dates and times
  13. RFC 1738 “Uniform Resource Locators (URL)”, available at <https://tools.ietf.org/html/rfc1738>
  14. RFC 2396 “Uniform Resource Identifiers (URI): Generic Syntax”, available at <https://tools.ietf.org/html/rfc2396>
  15. RFC 3629 “UTF-8, a transformation format of ISO 10646”, available at <https://tools.ietf.org/html/rfc3629>
  16. RFC 7231 “Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content”, available at <https://tools.ietf.org/html/rfc7231>

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